Well on old school Chevys, beginning with the Gen 2, 90 deg engines, if the center exhaust ports have a big space in between , it's a big block. It's a "bigger" block.( To confuse the issue the Gen 1 "W" big blocks, 348, 409 and 427.1, had the center two ports close together.) Cu in is NOT the determining factor. The pistons are spaced out for usually larger bores. The valve covers are much bigger. If the two center exhaust ports are close together, it's a small block, Of course, there are other differences. But you can see the intent by looking at the exhaust manifold up until the LS motors. The distinction is getting more and more blurred. The LS9 (known as a small block) has 638 HP out of the box and has "spread" exhaust/spread bores. Corvette is the platform for these big small blocks. Aluminum engines. When you design big bores and need strength, you have larger bore spacing. When you do that and you have super high horse power, the word "small block" has lost it's meaning almost. Big block boils down to a larger block, sort of. BBC started out being shorter but wider, that is no longer the case either. Totally Nodular dude!
BTW: most Chevy big blocks and small blocks have the SAME tranny bolt pattern. There is a BOP bolt pattern also.
