Is there a junkyard gem slant six combo?

i don't know the magic recipe of which head is the best, which bottom end has the most compression and/or best pistons and rods, but that seems to be the direction you're heading which is excellent.

i will however strongly encourage you to run something other than the 1920's, anything other than them suckers. or at least build your manifold so that it can accept another carb when you tire of fussin' and fightin' with them god forsaken things.