Another vote for sticking with the 400. Worst case, the easiest swap would be a Pontiac 455. You can use all your accessory brackets, accessories, motor mounts, and tranny (beef it up though if you're putting serious power to it). Not to mention your air cleaner and shaker can be reused as long as you stick with the factory intake or use an Edelbrock Performer (NOT the RPM) intake.
What I did was pull my numbers matching block, put it in the basement for safe keeping, and dropped a Pontiac 455 bored 60 over in between the fenders. I can flog the 455 if I want and should it break, I can still put the original numbers matching engine back in. It's only numbers matching once in its life.