Max MPG- slant vs 318/340

Lets discuss MPG... I see guys with slants talking about getting somewhere around 20-22 or so with good tuning, and I wonder what peeps are getting with a mild, sharply tuned 318 LA or even magnum. Sorta thinking a good 318 magnum, low gear A999 lockup trans and a 2.94 gear in a A body could possibly get better milage than the slant..... Thoughts?