building a 67 firebird and have narrowed my motor choices down to a 400 or a 455. in both cases i would use aftermarket intake,heads, pistons, cams ect, so stock compression is not an issue. does the 455 have enough more potential to justift it over the standard 400 block. also, is there a difference between the 400's in the gto and the firebird?

There's no substitute for cubic inches. All Pontiac blocks(not corporate GM) from the 326 to the 455 have the same outside dimensions. Don't believe me? Look it up. Please look before you say I'm an idiot. If i'm wrong, fine, I'm an idiot. So the only diffs between GTO and Birds would be manifolds, brackets, the basic bolt ons.