Reason the bolt holes on the front of the block differ is the BBC was never installed in a 1955-'57 Chevy, or a 1955-'64 Corvette that used four motor mounts to hold the engine in the frame (two up front and two in the rear that bolted to the cast iron bell housing). The BBC was used in trucks that always used four motor mounts but the spacing was different (and trucks used a 14 inch clutch disc instead of an 11 inch clutch so the rear bolt spacing also differed). Cars went to to the now common two middle of the block motor mounts and one at the rear of the tranny in 1958.
That bolt spacing difference caused me to go nuts trying to bolt a SBC onto my test engine stand until I got out a ruler and measured the difference. Had to weld up a separate set of front engine mounts to bolt a SBC into my engine stand that had used the four mounts and 14 inch bell housing off of an old dump truck to put a SBC motor on my test stand. (BBC is two inches longer than a SBC so the mounts had to go back and an adapter made to hold the SBC motor in place on a stand designed for a truck engined BBC).
Can't harp on that enough. People don't realize that every GM vehicle that had a 455 used the same size block for their 350. You can not tell a 455 big block Poncho from a 321 small block, or a Buick 455 from the smaller displacement 350 by looking at it. Same for the Olds rocket 455 and 350. Cadillac never had a small block, only big blocks, and bigger blocks. As to the big block Caddy 500 cubic inch production engine was still only a little bigger and about ninety pounds heavier than a SBC (the 455 Buick only adds twenty pounds of extra weight and is the same size as the SBC).
Not one of GM's big blocks can compare to the mass of a Chevy BBC because it was designed for use as a truck engine. Gasoline powered truck engines run at wide open throttle. All day, every day, for 300,000 miles between rebuilds. As such they have to be heavy, and strong.
That is why a BBC is two inches longer than a SBC (less room in front of the water pump for a fan blade) and two inches taller (less room under the hood; so good luck closing a flat hood if the car didn't offer a BBC as an option) and an inch and half wider at the valve covers (don't forget those big cast iron exhaust manifolds) so it doesn't fit in an economy car like the Nova or a Camaro without sitting side saddle. This is not to mention the added 220 pounds of cast iron over the front axle that a SBC powered car doesn't have to deal with. If you think of it like you do an 1,100 pound L6 Cummins diesel it makes sense. Big, strong, and very heavy.