Max MPG- slant vs 318/340

I would even go so far as to say, having a fuel injected 318 (stock truck motor) would be even cheaper than trying to fuel inject a slant... I guess what I am saying is the slant seems like a lose/lose situation, as far as milage, unless you just want to keep your slant for the sake of keeping it. I wonder in my hypothetical car... 318 magnum, headers, good exhaust, low gear 904 (A999) and a 2.94 or so gear in a 3100 lb A body, just how high you get get the milage....25-28 possible, or perhaps more????