I did a little math, that is to say my wife, the math teacher, did a little math for me. The difference in width between the RB manifold and the B manifold is 1.06 inches. So to fit an RB manifold on the B block would require the removal of approximately .53 inches of material from each side of the manifold.

Get a second opinion on the numbers before you cut. I've been known to be wrong before...once, but it happened.