Just curious as to why this thread doesnt exist. I know theres a lot of these swaps on several G's here, even though its not as popular or efficent as a more modern SBC, but a thread about it would be extremely valuable and answer a lot of simple questions that get reposted often. The swaps about SBC's are great....and are very similar, but there are a few differences w/the SBO swaps in our cars.
Im posting what I learned from my build....and those who have info to add, can and should add to or edit the following....thanks!
Motor Mounts
You will need to change the v-6 mounts to 307 mounts from an olds G-body car. They are not clam shells. They mount on the back holes towards the firewall and are side specific, right and left are different. You will need 307 motor mounts for your engine, which raise it up higher than the stock 350 mounts, because the crossmember in an older cutlass/olds is wider and the motor sits lower in the chassis.
Fuel lines
3.8L Buicks have the fuel pumps on the drivers side and the fuel lines are run along that side to accomodate it. You will have to reroute the fuel lines over to the passenger side. It is IMPORTANT that this is done correctly because once the motor is in, its nearly impossible to alter them later. I recommend you bend the new lines to follow the crossmember and pass through the holes cut out for the lines on the otherside. Its a pain, but if you use rubber line, replacing them down the road will be even harded to do. Depending on the fuel pump you use, you may have to run the return line as well. I used the stock pump from the 1970 model engine, so there was no return line to worry about. The vent line from the tank that ran to the charcoal cannister needs to be capped off or crimped (if you dont plan to reuse the canister), or it needs to be relocated to the passenger side of the engine bay where the battery used to be. I capped the line and deleted my canister for the time being and I drilled a vent hole into the fuel cap to equalize pressure from the tank.
Battery
The battery on 3.8L cars is located on the passengers side because the starter on 3.8's is also on the passengers side of the engine. To simplify things and to make it appear more stock, you need to relocate the battery to the drivers side, where the canister resides normally. This is fairly easy to do, but does require some modifications to the inner fender well that has mounts for the canister, which have to be removed so the battery will sit flat. The battery hold down will work on that side just as it did on the other side.
ECM
If you are not planning to use a CCC carb set up, the ECM will be rendered useless and should be removed from the car to clean up the wiring a bit. This will require the passengers side fender to be loosened at the back near the door which gives access to the lower firewall area. By pulling out the ecm and wiring all together, it can be reused if needed at a later date. A patch needs to be fabricated to cover the hole(s) that the wires left to keep out water and fumes.
"I would like to point out that if one wants to run the CCC on a SBO then they will need to sawp out the V6 CCC harness and ECM for a 307 CCC harness and ECM. Each engine type has it's own special CCC setup that doesn't work well on other types. For example when I swapped my 3.8 V6 out for a Chevy HO 305 I had to also swap out the V6 harness and ECM for a SBC harness and ECM, it was pretty easy to do though. Also the setups of the CCC systems were changed some in later year models such as Olds getting a knock sensor much later than the other brands. It would be a good idea to retune the 307 carb to run on a larger 350" .... (added by a helpful member)
Transmission Crossmember
Depending on the transmission and exhaust system being used, you may have to change the crossmember. I use a G-Force crossmember for a 200-4R transmission which has dual exhaust humps and is a stronger unit than stock. Most 3.8L's came with the infamous TH200c transmissions, which will not hold up long to the torque of a 350 SBO. You can modify the stock crossmember to allow for dual exhausts and a TH350, but it takes a good bit of accurate cutting and welding for the cutout. The trans mount is a direct fit for a th350. I used the 200-4R because I wanted better gas mileage. The driveshaft will work with both transmissions.
Suspension
Depending on what model cutlass you own, you either have the more common standard cutlass soft ride or the optional F41 sport suspension, found on 442's, H/O's, and all Salon models. The soft suspension coils will not like the heavier SBO motor. I recommend swapping in the 307 or 350 diesel springs, F41 sway bar, and upgrading the sway bar link ends/frame mount bushings to poly urethane from Energy Suspension. Also, better shocks wouldnt hurt either. Of course, all of this depends on you plans for the car, which will alter the suspension plans. I use mine as a driver, cruiser....so I want it to handle and ride smooth. Now would also be a good time to check/repair/upgrade/replace any brake components because the car will not be the same after this swap...it will run a lot faster, quicker, and handle differently than it did with the lower powered, lighter weight, v-6.
Wiring of the engine, gauges, alternator, starter, etc.....
If you are using a stock rally gauge cluster, there are some wiring issues to overcome. If you are not using a rally cluster, your wiring issues are far easier to handle. Since I had a rally cluster, I used a temp sender for a 77 350sbo and it works fine. It has one terminal for the gauge. But if you need a two terminal sender, use a 307 unit with an adapter to fit the older manifold threaded hole. I used the same oil sender as the 3.8 has and I made a tee to add the pressure sensor for the oil idiot light on the dash. This sensor came from a 307 sbo. You dont have to add this, but if you happen to lose oil pressure, you wont know it till its too late. Also, this sender is needed for the ecm, if you are going to use it. I just added it for peace of mind. You will find the tee and/or the extension needed to allow the pressure sender to work on this engine at Napa. 1/8th NPT is the size. If you go with the sender only, use the 45 degree elbow that napa sells which will thread into the block and allow the sender to thread into the elbow. The voltage gauge is pretty simple, using the same wiring as before. If the alternator, starter, and battery are wired up correctly, the gauge should work fine and the car should start. It cannot be stated enough.... make sure the ground strap is properly connected to the firewall and the battery negative cable has a ground to the body. Too often during swaps, these are forgotten, and either the motor wont start, or the alternator/electrical system has issues. Use good wires for the battery. You can have them made for you at Orielys or Napa, or you can buy some at AZ, but make sure the ground wire is at least a 2 gauge!
Radiator, shroud, and top hold down plate..........
These are not reusable and must be obtained from a 307/305 gbody. The 3.8 shroud is too small, as is the radiator, and the top plate doesnt exist on the v-6 cars. If you would like to keep appearances stock, use a 307 shroud and top plate, and a good quality stock radiator (cu162). Depending on the water pump you have on the SBO 350, you may run into issues with the bottom radiator hose. Most of them have a 1.75inch inlet on the bottom of the water pump, so you can use a 307 bottom radiator hose. But, if you have a 2inch inlet pump, such as w/a non-a/c set up, you may have to fabricate a hose to fit, because there isnt a hose made, at least not that I can find, with the correct size openings to fit in a gbody using the 350 non/ac waterpump. For the top hose, use a stock 307 unit. Obviously, you have lots of choices on mods in this area to suit your tastes, such as electric fans, custom aluminum radiators, custom shrouds and even custom top plates. In the end, I opted for a stock appearance along with the quieter and more reliable clutch fan setup.
Im posting what I learned from my build....and those who have info to add, can and should add to or edit the following....thanks!
Motor Mounts
You will need to change the v-6 mounts to 307 mounts from an olds G-body car. They are not clam shells. They mount on the back holes towards the firewall and are side specific, right and left are different. You will need 307 motor mounts for your engine, which raise it up higher than the stock 350 mounts, because the crossmember in an older cutlass/olds is wider and the motor sits lower in the chassis.
Fuel lines
3.8L Buicks have the fuel pumps on the drivers side and the fuel lines are run along that side to accomodate it. You will have to reroute the fuel lines over to the passenger side. It is IMPORTANT that this is done correctly because once the motor is in, its nearly impossible to alter them later. I recommend you bend the new lines to follow the crossmember and pass through the holes cut out for the lines on the otherside. Its a pain, but if you use rubber line, replacing them down the road will be even harded to do. Depending on the fuel pump you use, you may have to run the return line as well. I used the stock pump from the 1970 model engine, so there was no return line to worry about. The vent line from the tank that ran to the charcoal cannister needs to be capped off or crimped (if you dont plan to reuse the canister), or it needs to be relocated to the passenger side of the engine bay where the battery used to be. I capped the line and deleted my canister for the time being and I drilled a vent hole into the fuel cap to equalize pressure from the tank.
Battery
The battery on 3.8L cars is located on the passengers side because the starter on 3.8's is also on the passengers side of the engine. To simplify things and to make it appear more stock, you need to relocate the battery to the drivers side, where the canister resides normally. This is fairly easy to do, but does require some modifications to the inner fender well that has mounts for the canister, which have to be removed so the battery will sit flat. The battery hold down will work on that side just as it did on the other side.
ECM
If you are not planning to use a CCC carb set up, the ECM will be rendered useless and should be removed from the car to clean up the wiring a bit. This will require the passengers side fender to be loosened at the back near the door which gives access to the lower firewall area. By pulling out the ecm and wiring all together, it can be reused if needed at a later date. A patch needs to be fabricated to cover the hole(s) that the wires left to keep out water and fumes.
"I would like to point out that if one wants to run the CCC on a SBO then they will need to sawp out the V6 CCC harness and ECM for a 307 CCC harness and ECM. Each engine type has it's own special CCC setup that doesn't work well on other types. For example when I swapped my 3.8 V6 out for a Chevy HO 305 I had to also swap out the V6 harness and ECM for a SBC harness and ECM, it was pretty easy to do though. Also the setups of the CCC systems were changed some in later year models such as Olds getting a knock sensor much later than the other brands. It would be a good idea to retune the 307 carb to run on a larger 350" .... (added by a helpful member)
Transmission Crossmember
Depending on the transmission and exhaust system being used, you may have to change the crossmember. I use a G-Force crossmember for a 200-4R transmission which has dual exhaust humps and is a stronger unit than stock. Most 3.8L's came with the infamous TH200c transmissions, which will not hold up long to the torque of a 350 SBO. You can modify the stock crossmember to allow for dual exhausts and a TH350, but it takes a good bit of accurate cutting and welding for the cutout. The trans mount is a direct fit for a th350. I used the 200-4R because I wanted better gas mileage. The driveshaft will work with both transmissions.
Suspension
Depending on what model cutlass you own, you either have the more common standard cutlass soft ride or the optional F41 sport suspension, found on 442's, H/O's, and all Salon models. The soft suspension coils will not like the heavier SBO motor. I recommend swapping in the 307 or 350 diesel springs, F41 sway bar, and upgrading the sway bar link ends/frame mount bushings to poly urethane from Energy Suspension. Also, better shocks wouldnt hurt either. Of course, all of this depends on you plans for the car, which will alter the suspension plans. I use mine as a driver, cruiser....so I want it to handle and ride smooth. Now would also be a good time to check/repair/upgrade/replace any brake components because the car will not be the same after this swap...it will run a lot faster, quicker, and handle differently than it did with the lower powered, lighter weight, v-6.
Wiring of the engine, gauges, alternator, starter, etc.....
If you are using a stock rally gauge cluster, there are some wiring issues to overcome. If you are not using a rally cluster, your wiring issues are far easier to handle. Since I had a rally cluster, I used a temp sender for a 77 350sbo and it works fine. It has one terminal for the gauge. But if you need a two terminal sender, use a 307 unit with an adapter to fit the older manifold threaded hole. I used the same oil sender as the 3.8 has and I made a tee to add the pressure sensor for the oil idiot light on the dash. This sensor came from a 307 sbo. You dont have to add this, but if you happen to lose oil pressure, you wont know it till its too late. Also, this sender is needed for the ecm, if you are going to use it. I just added it for peace of mind. You will find the tee and/or the extension needed to allow the pressure sender to work on this engine at Napa. 1/8th NPT is the size. If you go with the sender only, use the 45 degree elbow that napa sells which will thread into the block and allow the sender to thread into the elbow. The voltage gauge is pretty simple, using the same wiring as before. If the alternator, starter, and battery are wired up correctly, the gauge should work fine and the car should start. It cannot be stated enough.... make sure the ground strap is properly connected to the firewall and the battery negative cable has a ground to the body. Too often during swaps, these are forgotten, and either the motor wont start, or the alternator/electrical system has issues. Use good wires for the battery. You can have them made for you at Orielys or Napa, or you can buy some at AZ, but make sure the ground wire is at least a 2 gauge!
Radiator, shroud, and top hold down plate..........
These are not reusable and must be obtained from a 307/305 gbody. The 3.8 shroud is too small, as is the radiator, and the top plate doesnt exist on the v-6 cars. If you would like to keep appearances stock, use a 307 shroud and top plate, and a good quality stock radiator (cu162). Depending on the water pump you have on the SBO 350, you may run into issues with the bottom radiator hose. Most of them have a 1.75inch inlet on the bottom of the water pump, so you can use a 307 bottom radiator hose. But, if you have a 2inch inlet pump, such as w/a non-a/c set up, you may have to fabricate a hose to fit, because there isnt a hose made, at least not that I can find, with the correct size openings to fit in a gbody using the 350 non/ac waterpump. For the top hose, use a stock 307 unit. Obviously, you have lots of choices on mods in this area to suit your tastes, such as electric fans, custom aluminum radiators, custom shrouds and even custom top plates. In the end, I opted for a stock appearance along with the quieter and more reliable clutch fan setup.
