I'm looking to start a project with a 77 or 78 trans am..?

the 455 will give you more power. you can use any 455 as long as it,s out of a pontiac, oldsmobile or buick.

Bigger never hurts. On a moderate build every inch can be 1 horse. If you can get either engine and you want the most power might as well get the big one.

What engine is in the car now? I think there were 3 options, you could have had a Pontiac, Oldsmobile, or I think for some of the CA emissions cars even a Chev motor. It would be easiest to do the swap staying with the same type of engine that is in there now. The Pontiac engines were all basically the same on the outside so anything from a 265 to a 455 will pretty much bolt up. Oldsmobile the basic blocks were the same but the 'big' motors like the 425 and 455 had a taller deck so they were slightly taller and wider than the 350 or 403. All those motors used the same transmission mount. The Chev had different blocks. There was a small block that went up to 400 and a big block that went from a 326? up to now the 572 and within the big block there were tall and short deck versions. Chevs also had their own transmission mount.

Anything that is not a Chev engine is getting harder to find and more expensive to build. Plan on spending at least twice as much for a Pontiac or Olds. The Olds 350 and 403 was used in many cars 77-79 so they are not that hard to find. Pontiac V8's got pretty rare after 76 so they are much harder to come by. Pontiac's last V8 was 81. Oldsmobile's went into many RWD cars in the earlier 80's and were used in some models till 90. Since the basic blocks were the same the whole time there are a few more of the basic parts around.

455

ya can't beat cubic inches.