Mike P and R Pope are right on the money. Put a Caddy back in it, either the stock year or a 472-500.

Bulletproof, powerful, and they fill up the engine bay. I personally hate to look into someones big Caddy , Olds, or Buick and see a tiny 350 Chev in there. Looks lost. I realize it is sensible, but......

Gas milage isn't going to be great regardless of what you install.....theres alot of iron to move there.

Good luck.