slant 6 big valves

The go-to source for good big valves for a Slant-6 is Engnbldr; here is a set of the exhausts—give 'em a call on 503-257-7947 for the intakes.

(Be very careful before you decide to give any money to Clifford. They have a long and ugly reputation for being a bunch of clowns; see for example here, here, here, here, and here. Much of what they sell is inaccurately described, and a lot of it is not even slightly cost-effective. The good news is that you don't need to go to Clifford to get hot rod parts for slant-6s; there are lots of other, better options. See for example Dutra Duals and header options discussed in this thread and this one, Erson custom cams, HEI ignition upgrade, Mike Jeffreys windage trays, Hurricane intakes, other exotic intakes. Hi-perf engine buildup here, high-perf parts and build info here)