B and RB have no bearing on whether the cranks will fit due to stroke. Different displacements in different blocks with different rods will require different things. GOing of an iffy memory.. the strokes longer than 4.15" in a B block with the long rod (7.1) need the oil ring supprts. RB does not. The 4.15 stroke clears the oil pickup boss in the RB with any rod. The 4.25 needs the 7.1" Chevy pin size rod to clear, and it is VERY close so you may still need to trim it.
And, LIke John said. No block is anythicker than the others. Not by year, not by casting number, not by deck height. All suffer from core shift which is the great equalizer that saps potential from them all. Always have a block sonic tested when exceeding the factory stroke. A sleeve or two and the test is still far cheaper than splitting a block, hydraulicing it and bending or breaking things.