My first look when needing an engine, or major upgrading is to look for a doner car. Depending on your situation, I would look for a rough bodied/wrecked Caddy with a 472/500/425 with attached auto. If you have the skills and tools it is possible to upgrade to a Chev. Remember, the longer you have this car (no matter which drive train used) the fewer parts will be available. So decide if shortterm or longterm before swapping.