78 Cutlass factory 305 to 71 455 olds

[Carolinadoublej]

after doing a search, most of what I’m seeing is the newer gbody engine swaps. I’m trying to nail down which mounts that I need to mount this engine to the frame rail. I did a similar swap in a 78 cutlass but it had the factory 260, but this being a factory 305 has me scatter brained. Any part #s for the pads and engine mounts that I’ll need? It has the th350m that I’ll be using behind the 455. Transmission guy says that it’ll bolt up and run behind the 455.

 

[Doug Chahoy]

I’ve always heard that you’d need an available adapter plate for a Chey trans to a BOP. And vise versa. Good choice on the engine, 71s still had good power yet run on unleaded gas. You could leave it stock and have alot of fun with it.

 

[Carolinadoublej]

after doing a search, most of what I’m seeing is the newer gbody engine swaps. I’m trying to nail down which mounts that I need to mount this engine to the frame rail. I did a similar swap in a 78 cutlass but it had the factory 260, but this being a factory 305 has me scatter brained. Any part #s for the pads and engine mounts that I’ll need? It has the th350m that I’ll be using behind the 455. Transmission guy says that it’ll bolt up and run behind the 455.

F

I’ve always heard that you’d need an available adapter plate for a Chey trans to a BOP. And vise versa. Good choice on the engine, 71s still had good power yet run on unleaded gas. You could leave it stock and have alot of fun with it.

I have heard the same thing but he assured me it was a th350m meaning multifit. We will see I guess. The whole motor mount deal has me stumped... I’ve seen where the guys in the newer gbody were using the mounts off an olds 307 motor and frame, but don’t know for sure if that’ll work for a 78

 

[Doug Chahoy]

From what I’ve read on here I believe your right. The Olds experts will jump on this thread soon and fill you in on exactly what works.

 

[Red78Calais4speed]

I replaced the Olds 350 in my 1979 Hurst with an Olds 455 out of a 1971 Delta 88 and I used the motor mounts from the 350 with no issues. The motor sat in the chassis where it was supposed to without any problems. I was told by 4 different "experts" that the heater box with the AC components wouldn't fit so I removed them and used a block-off plate. I was later told by several other people that they'd done this swap before and that the heater box could stay but there was only about a 1/2 to 3/4 inch clearance on the passenger side near the box. Last year I took out the 455 and put a 1969 vintage Olds 350 back in along with the heater box and AC components. I wanted to go back to completely stock. This spring I Just need to pull the 8.5 inch 3.73 posi rear out and put the 7.5 inch 2.73 posi back in. The 455 was great but I got 8-10 mpg with the 3.73s, turned 3400 rpm at highway speeds (60-65), and was always plagued by cooling issues. Probably wouldn't have been too bad if I left the heater box in.
In regard to your transmission, you'll probably need an adapter plate. I don't think the multifit cases were used much in the early years (78-80) so it probably has just the Chevy bolt pattern. Depending on how much power you're making, that TH350 might not live behind the big block. I'm on my second one now. The first one let go about a year after I installed the 455. I had a local transmission expert build one for me that he claimed would be good up to 750hp. It's been in the car since 2001 without the single issue. It'll still chirp the tires shifting into second gear even with the Olds 350 re-installed.

 

[pagrunt]

Depnding on the budget you have (or might plan for down the road to have) look into a TH200 4R since they have both patterns. Should be able to get one built to handle the 455. Just need to work in what is needed for the lock up converter & a trans crossmember.

 

[69hurstolds]

A TH350 should live with a street built 455 as long as you're not flogging on it all the time. You could build the trans up to handle more torque, however. There's a reason they put TH400s behind 455s. Check the case physically to ensure it's a dual bolt pattern, though. If not, you'll need an adapter plate.

 

[Carolinadoublej]

I appreciate the info guys... what am I up against besides just the engine mounts?

 

[pagrunt]

Should be able to get away with any '78-88 Olds powered A/G mounts.

 

[Alerch]

I appreciate the info guys... what am I up against besides just the engine mounts?

Wiring harness will need to be modified, accessory brackets and the correct pulleys to go with the brackets, engine frame mounts, I did a big block Olds swap in my car a while back.. hindsight shoulda went with a LS but oh well that’s in the works 👍 any questions just do a quick search there is plenty of info on here to cover just about anything to do with these cars... a lot of knowledge on here!