http://www.socaljetboats.com/projects/455-swap-to-454/?PHPSESSID=i4a7lp2jpbbeab35qsm2vhuij1http://www.socaljetboats.com/engine-mechanical-electrical/glencoe-motor-swap/http://www.socaljetboats.com/engine-mechanical-electrical/455-olds-vs-454-chevy/Here are a few links from past threads. If you go to the home page and do a search for 455 swap, you’ll get a lot more info.
Basically to do the swap, you’ll need everything but the carb for a Chevy. Mounts, water pump block off, PTO (the piece that mounts the drive shaft to the flywheel), exhaust. The Chevy mounts and exhaust might not line up. Assuming you have through-transom exhaust, it’s a possibility you’ll have to figure out how to make the exhaust line up.
There was a great article in HotBoat magazine that is mentioned in one of the links above. But the link didn’t work. I’m pretty sure I still have the magazine. I’ll see if I can find it and scan it.
On the Olds, it might still be rebuildable. Just because it was rebuilt, doesn’t mean they removed a lot of material. Unless you have the paperwork detailing what the machinist did, the only way to tell would be to take it apart and mic everything. An Olds can be a great engine, with the right machine work and modifications. It’s always been a big debate when people mention swaps.
Do your research. It’s definitely a project. But not impossible.