I think with exhaust manifolds,a single exhaust of a large O.D. would be sufficient and probably help low end power. Headpipes into a 3" collector,a good muffler, and a 3" or maybe even 2.5" tailpipe. I had a mild cammed (.427/.454 lift)318 in my '77 Powerwagon,Performer intake and Holley 4bbl.I had duals for a while but went to single when they rotted away.Low end was much better,it had 3.55 gears and I revved the heck out of it.