Here's the way I understand it. The block size has more to do with the physical size of the engine than the cubic inch desplacement. Someone correct me if I'm wrong here...

Pontiac;
small block...326, 350
big block...347, 389, 400, 421, 455

Oldsmobile;
small block...307, 330, 350
big block...400, 425, 455

Ford;
Y small block...272, 292, 312
W small block...221, 289, 302, 351
FE big block...352, 360, 390, 406, 427, 428
385 big block...351, 429, 460

Chevy;
small block...265, 283, 302, 305, 327, 350, 400
old big block...348, 409
new big block...396, 427, 454

I know I left some out, but I can't remember them all.

MR