Wow. Didn't realize there was such passion about baseplates. I couldn't tell you about Chevy baseplates. I'm not a Q-jet expert. I've only had one spare from a 454 from a 1976. But none from a 305 CCC carb. The 2-point, 3-point, and 4-point adjustments for the carbs are about how to tweak them on the vehicle to tune them to final specs. In the end, the engine doesn't care how it gets fuel when it needs it, as long as it's getting what it needs. For the most part, the calibrations can be tweaked on any of the CCC carbs to get you where you need to be for the final application. AFAIK, most all E-jets are set as lean as possible for emissions purposes anyway. Primary jets/rods can be swapped out as well as secondary rods/hangers if needed, but from what I've seen, there's not much difference on primary jetting between the V8s. And ALL the E-jet secondary seats are the same. Just the rods are different. Also, for example, Olds has a 1/2 turn wrap setting on the secondary air valves, where the Chevy, I think, is nearly twice that. Ideally, you just want it not to bog when you get into the secondaries and you shouldn't "feel" the secondaries kick in. You may hear them, but you shouldn't feel them.
For the air horns, I've found Chevy and Olds are virtually the same in comparable year applications. I got a new Chevy air horn with the rich stop which could be used on the Olds 3-point carburetor. The only differences I've found are the filtered air port not present on the back of the Chevy one since the Chevy didn't need it for the E-choke, but can be drilled and fixed if needed for a hot air choke. I know, I did it. E-chokes can be put on an Olds (plug the vacuum port in the choke housing if you do) but it's harder to put on a hot air choke on a Chevy since it has no vacuum hole, you would have to drill one into the float bowl- yikes. If your air horn is a 2-point adjust, the rich stop screw hole is not drilled/threaded, and to use it on an Olds 3-point system (and some Mopars) you would have to drill and tap that. Another Yikes. Good luck.
The IABV "tower" holes where it actually pulls in air are in different locations, but again, that means nothing as Chevy's in the front above the IABV, and Olds is on the side. And the final difference was already mentioned. The Chevy used no air bleed restrictors for the main primary (as seen through the top of the air horn) where the Olds did. I used some well tube restrictors from my hosed up float bowl with smaller .030 holes and drilled them out to the proper hole size .047 before I realized you could actually buy these from the Q-jet shops.
FWIW, even the lettered IABVs are set with the tool and if needed, adjusted if needed. Typically the lettered IABVs are set with the tool and left alone and all the idle mixture is controlled with the mixture screws. But there's a caveat in the procedure that says if you can't get it exactly right with the mixture screws, you can tweak the lettered IABV until you get it right. With the non-lettered ones, you tweak both the IABV and idle mixture screws. WHY they did that stupid sh*t I'll never know. To that point, 305 in the 84 F-body specs says to set the IABV with the gage alone, suggesting they have lettered IABV. What's the difference between F-body carbs and G-body carbs for the 305? 🤷♂️
Ok, back to the subject at hand. Sorry I got derailed. Follow the ports. Match up the porting from your float bowl to the air horn, or from the float bowl to the baseplate, depending on what you're swapping. 95% of the time, they'll be the same. Check port SIZES as well. Just because they're there doesn't automatically mean they'll match up and be correct. going from tiny to big or big to tiny may make a difference. Check 'em with drill bits or small gage wires. I did that with my Frankencarb and the Chevy ports and Olds ports/tubes and all that junk matched right up. If there's an extra vacuum port you don't need- plug it. If there's a vacuum port you need that ain't there- if you're desperate enough- drill it. Just know what you're doing with it.
There's another thing you could do for your other carb and that's remove both baseplates, grind off the factory stakes in the primary throttle shaft blade screws, remove them, and then swap the primary shafts. It's a bit more work, but then it won't matter how the baseplates are set up. The throttle shafts are pretty much all the same on the CCC carbs, regardless of E or C models. The only difference is the throttle shaft main lever which is staked on to the end of the throttle shaft. NOT and easy fix to swap that. If Rochester would have figured out a way to attach those primary levers on without being pressed and staked, that would make carb swapping infinitely easier.
All in all, MOST of the time you can swap stuff over from carb to carb as long as you know which carb you have, adjustment-wise. From what I've seen, ALL E4ME/C carburetors have an M/C solenoid set by gage at 1.304" and a rich stop, if used, set at 4/32". 2-point carbs are built in rich stops, so no adjustment needed. Those two settings haven't changed. Also, all the IABVs are set with a gage, but some are "note 2" which says set with the gage, final adjustment on car.
For the air horns, I've found Chevy and Olds are virtually the same in comparable year applications. I got a new Chevy air horn with the rich stop which could be used on the Olds 3-point carburetor. The only differences I've found are the filtered air port not present on the back of the Chevy one since the Chevy didn't need it for the E-choke, but can be drilled and fixed if needed for a hot air choke. I know, I did it. E-chokes can be put on an Olds (plug the vacuum port in the choke housing if you do) but it's harder to put on a hot air choke on a Chevy since it has no vacuum hole, you would have to drill one into the float bowl- yikes. If your air horn is a 2-point adjust, the rich stop screw hole is not drilled/threaded, and to use it on an Olds 3-point system (and some Mopars) you would have to drill and tap that. Another Yikes. Good luck.
The IABV "tower" holes where it actually pulls in air are in different locations, but again, that means nothing as Chevy's in the front above the IABV, and Olds is on the side. And the final difference was already mentioned. The Chevy used no air bleed restrictors for the main primary (as seen through the top of the air horn) where the Olds did. I used some well tube restrictors from my hosed up float bowl with smaller .030 holes and drilled them out to the proper hole size .047 before I realized you could actually buy these from the Q-jet shops.
FWIW, even the lettered IABVs are set with the tool and if needed, adjusted if needed. Typically the lettered IABVs are set with the tool and left alone and all the idle mixture is controlled with the mixture screws. But there's a caveat in the procedure that says if you can't get it exactly right with the mixture screws, you can tweak the lettered IABV until you get it right. With the non-lettered ones, you tweak both the IABV and idle mixture screws. WHY they did that stupid sh*t I'll never know. To that point, 305 in the 84 F-body specs says to set the IABV with the gage alone, suggesting they have lettered IABV. What's the difference between F-body carbs and G-body carbs for the 305? 🤷♂️
Ok, back to the subject at hand. Sorry I got derailed. Follow the ports. Match up the porting from your float bowl to the air horn, or from the float bowl to the baseplate, depending on what you're swapping. 95% of the time, they'll be the same. Check port SIZES as well. Just because they're there doesn't automatically mean they'll match up and be correct. going from tiny to big or big to tiny may make a difference. Check 'em with drill bits or small gage wires. I did that with my Frankencarb and the Chevy ports and Olds ports/tubes and all that junk matched right up. If there's an extra vacuum port you don't need- plug it. If there's a vacuum port you need that ain't there- if you're desperate enough- drill it. Just know what you're doing with it.
There's another thing you could do for your other carb and that's remove both baseplates, grind off the factory stakes in the primary throttle shaft blade screws, remove them, and then swap the primary shafts. It's a bit more work, but then it won't matter how the baseplates are set up. The throttle shafts are pretty much all the same on the CCC carbs, regardless of E or C models. The only difference is the throttle shaft main lever which is staked on to the end of the throttle shaft. NOT and easy fix to swap that. If Rochester would have figured out a way to attach those primary levers on without being pressed and staked, that would make carb swapping infinitely easier.
All in all, MOST of the time you can swap stuff over from carb to carb as long as you know which carb you have, adjustment-wise. From what I've seen, ALL E4ME/C carburetors have an M/C solenoid set by gage at 1.304" and a rich stop, if used, set at 4/32". 2-point carbs are built in rich stops, so no adjustment needed. Those two settings haven't changed. Also, all the IABVs are set with a gage, but some are "note 2" which says set with the gage, final adjustment on car.