I found a nice Slant 6 article I would like to share.

No slant exhaust manifold has a 2 1/4 outlet. You can use an 2 1/4 flange with a 2 1/4 to 2 1/2 adapter pipe to build your own 2 1/2 exhaust pipe.

Replacing a worn out 48 year old distributor, with a new one may produce the same results. The Davis DUI distributor has a built in 4 pin HEI, so the HEI upgrade should produce similar results.