I recently talked to an engine builder who built a 496 mudd-bogg engine, 14:1 compression, he used a comp cams 236/242 .612/.612 @112 lsa, it has only 15 deg overlap. He got 21 inch vacuum. He uses total seal rings, said that adds 1-2 in Hg, bleed down hydraulic roller lifters add another 1-2 (similar to rhoads lifters). He told me this cam will make around 16-inch vacuum on my street 540. His engine made 625 horse.