There are eight different big blocks. The B blocks are short deck and the RBs are tall deck. The RBs require a wider intake manifold.
B: 350, 361, 383, 400
RB: 383, 413, 426 Wedge (not Hemi), 440
All B engine use 3.38 stroke crank with different bores, and all RB engines use 3.75 stroke crank...