20mpg is pretty darned good!

The larger valves/ports of the 360 heads are definitely more efficient than the 318 heads.

I did a 318 with the 1.88 intake 360 heads and it easily broke into the 14 second range @ 95mph at the track in a street driven car. I got 17 mpg but had big single plane intake, headers, .480 Comp cam, 3.23 gear etc.