Nope,
not the same blocks. A friend used to also say, all standard GM 350's. Not true.
Big difference in crank shaft bearing journal sizes. Some had a taller deck, for a longer stroke. Some remained over square, so had a larger bore, requiring a wider, longer block.
The small block Chevy maxed out at 350 cubic inches, and then a siamesed cylinder 400, unless a custom builder made it into a 383 stroker engine
A 396, 427, and 454 were bigger MkIV blocks.
Small block marketing designation was "turbo fire" Big block was "turbo jet.
Across GM divisons, each cast their own block, at least until the mid 70's. Then the main difference were the heads, manifolds, and belt drive accessory mounting.
An Olds engine had positive valve rotation first, and it's distributor rotated in the opposite direction from the others. They ran very smooth, and sounded different.
Chevy had it's starter on the opposite side, so the battery also, and later, the A/C compressor on the drivers side. The others were on the passenger side, and Cadillac in the center with the upper radiator hose circling over it.
So all the drilling in the blocks were different.
Buick as you may well know, had it's distributor up front on an angle, same for the oil filter. The others behind the oil pan side.