My question is simple. I have a 1980 Cutlass Supreme and a 700r4 rebuilt transmission. Will a 1975 olds 455 fit in this 1980 G-body?

 

Been there, done that.

Mine sortta went like this.....

260....350 (3 rear ends)...455...350. All on the same mounts and through the stock exhaust.

 

What's in the car now? If it originally had an Oldsmobile V8, the 455 will drop in like it was made for it. If it had a Buick V6 or Chevy V8, you'll have to change some things, like wiring, accessory brackets, all those little things that will trip you up.

700 will not bolt to an Olds engine without some type of adapter plate. It's a tough unit, but make sure it's built to spec. Rear end may be a light-duty piece too.

Last, 1980 is still an A-body. The G-body platform did not appear until 1982.

 

Beat me to it. I've seen those adapet plates on Summit, about $100 or so

 

Can you clarify that some more? I thought '78 and newer were g-bodies.
At the very least '81 & '82 were pretty much the same car.

 

The frames from 78-88 were the same, but the designator was changed in 1982 because they introduced the new FWD platform that year and wanted to call that the A body. So GM in their infinite wisdom changed the mid sized RWD platform to G body.
So the mid size was the A body until 1981 when that moniker was moved to the new FWD platform and the new G body moniker was added the the mid size RWD platform in 1982 to replace it.