what about a crate motor?
GM crates are all Chevy so he would need a new transmission, Two patterns Chevy and BOP I believe.
As far as 455 goes I am not an Olds expert but I believe the 307, 350 and the 403 share the block while the 455 is out there with teh 425. There is no noticeable difference in physical size. The 403 is a good motor too and would be a straight bolt in. Remember that Pontiac used this in the Trans Am andsome Can Am's.