If it's going into an A body the 3/4" shorter V of a B block is much much better fit from just the shorter block. As said you do have the crank but you also said 440 headers so will these headers hit something on a B block? If so I'd go w the 440 & use the RB specific headers/sell the 451 crank and cube wise you can go higher on the 440 until CH/piston rock becomes a prob for longevity