Ok, many times rebuilt engines actually are better than new. Most GMs will fit other GMs, most ford will fit other Fords. SB and BB have to do with the amount of material on the block. Therefore, you can get more displacement from a BB than a small block. New engines are good, but crate engines are worth the money if you have it. Crate engine being HP already made. Components are usually only swappable between same make engines. Pontiac to Pontiac, Chevy to Chevy, Olds to Olds. Hope this helps, and if you need to know more, just ask.