a chevy 396 is a big block but a 400 chevy is a small block..
I lost a bet on that one before,
You could have won it on a technicality. in '70-'72, there were actually two engines sold as a "400". The "Turbo fire 400" was a small block, and generally found in passenger cars, mostly Impala Wagons with the tow package. In trucks, one could get the "Turbo Jet 400", which was actually a 402 big block. I believe that one could also be optioned in Passenger cars as a "400" but is more common in trucks. The 402 was used in the Camaro, Chevelle, and Nova, but marketed as a "396".
Now, to answer the basic questions, all small blocks share the same external dimensions, most the physical differences are internal. The displacements are as follows; 262, 265, 267, 283, 302, 305, 307, 327, 350, 377 (race use only, mostly in Grand Sport Corvette), and the 400. These are factory blocks, not counting any aftermarket blocks like World Products.
Wit big blocks, there are several variations, the Mark 1, Mark 2, Mark 3 (not produced), Mark 4 (most common), Gen 5, and Gen 6. There was also a variation that was for all intents, a big block version of the LS engine. The big block sizes are as follows; Mark 1 348, 409, 427. Mark 2 427, aka, the "Mystery Motor"-race use only. Mark 3, unknown, none released to the public. Mark 4 Short Deck 396, 427, 454. Mark 4 Tall deck 366, 427. Mark 4 Aluminum block. 427 (ZL1 with iron liners), 430, 465, 495, 510 (the last 4 listed are 390 alloy, no liners, race only). Mark 5 454. Mark 6 427, 454, 502 (tall deck) 572 (tall deck).
Again, I only listed blocks manufactured by GM. All V Chevrolet engines, including the 229 and 262 V6 and the 194, 230, 250, and 292 inline engines use the same bell housing bolt pattern.
When you get into the other GM brands, that opens a whole different can of worms, especially Pontiac, which had no actual "big block" engine.
