[MrPbody]

Through '69, all GTO and Firbird blocks utilize the "2-bolt" mounting flange. '70 through mid '76, all blocks had 5 mounting holes, making them interchangeable across "all" years. Mid-'76 and newer have but three holes in the block. This applies to the 400. Virtually ALL 455s were "5-bolt". The mounts use either 3 or 2 bolts. None use all 5.
The "best" blocks (subjective criteria) are the '70-'mid '76 blocks with 5 mounting holes. Not because they fit all the eras, but because there's more "meat" in the bottom of the bore, making them better candidates for performance builds.
Ames Performance, Performance Years, Indian Adventures, plus a few others, sell an adapter kit to cross eras with blocks. Call them and tell them which block you want to put in which car, and they can send you ther appropriate stuff.
Jim Hand's book "How to Build Max-performance Pontiac V8s" has most of this info in it. Good reading. Necessary reading if you haven't built a Pontiac lately...