I love Olds engines and we built a 468 ci 455 for my Sons 30 RPU. We are running a 350 turbo and they are available in the Buick Olds Pontiac bolt pattern as you mentioned, or, you can buy a $ 75.00 adapter and use the Chevy bolt pattern 350 (or 400) I think you might have less underfloor clearance issues with the 350 than the 400..........but that is just a guess.

One problem area is that the 455 has the starter on the drivers side, so it might interfere with steering, if the box is over there (not familiar with your model). Motor mounts are common side mounts, so simple frame brackets would probably get those adapted. You will need a new transmission mount and driveshaft built.

The early 455's are the good ones and they started getting smogged around 72 or so, but were still good motors, just not as much HP.

Don