The height of the block V is 3/4" (.745) taller on the RB. If my math is right using the V increase as the hypotenuse of a right triangle then the height increase is going to be .745 divided by 1.414 for a .527 vertical height increase with all of the bolt ons being the EXACT same dimentions & this lets you go with a longer stroke with the RB rod till the CH of the piston gets so short to where there are issues. I think you can get "buttons" to cheat a bit on that so the pin is in the oil ring but I would avoid that if possible. I believe the B block is stouter & I'd always thought they were lighter but I was thinking someone said that in some cases that ain't always true. In an A body use the B hand down. Anything else, if you can get the cubes you need with an RB rod/no piston problems then I would go with a B (~500 cubes or a bit less) & if I needed more than that then I'd suggest starting with the RB block & going with the max cubes. People have went way above 500 in the B but this is my preference.