The Buick install is a snap, frame stands and motor mounts and you're there. BUT, for gods sake as you build l
keep an eye out for a Pontiac 400 or 428. The car will be worth more with a Pontiac in it. Plus, the Pontiac crowd tend to be diehards as far as engine choice is concerned.

If you find a 455 cheap, it's only a crank & piston change to make it a 428.

Good luck, that looks like a worthwhile project.