Gas mileage is in the compression....trick is to stay conservative on the cam and advance the piss out of it....

My last 350 easily knocked down 22mpg loaded or not with a 4bbl...I used flat tops and a .005 deck height 882 heads with the stock valves and very light port work a 327hp cam advanced 8deg.... total time of 40 with 92 octane 4.56 rears with standard size 1-ton tire (275-60r-16's)

My 4x4 had a 400cu and got the best of 24mpg with same deck and 10cc dish pistons diff cam same advance and 4.10 gears on 38x15-50's MT's

do your home work....it can be done.... shoot for a "true" 9.5-9.8 compression or go to reverse cooling and you can lean on 10.2