The subframes on the post 69 Camaro/Firebirds are front steer like the Chevelle and GtO frames. So you may have to fabricate frame mounts for the early rear steer frame. A 455 Olds will fit. As mentioned you will need an Olds compatible trans but any BOP 350/400 turbo should fit. exhaust manifolds or headers might be tuff to find also. As the steering box gets in the way.
I have done 500 Caddys in this combination with nearly all factory parts.