Your exhaust guy is right, the smaller pipe should give you more torque. But at some point it starts restricting power on the top end, you mentioned that a cam is next??? With the 4 barrel, the offy, the HEI, and a little more cam I would think you would want the whole 2 1/2". On the other hand, if you don't plan on spinning it any faster than stock you may want the 2 1/4".
