My 86 Mustang GT 5.0HO got 25-27 mpg over a 700 mile trip (three tanks of gas). heads on the 86 are very 'closed'. If I were building another 5.0 (and I will be in the next year or so), I'd use the F7TE closed chamber heads, flat-top no notch pistons, zero decked. I'd use the stock Mustang intake or the Holley SysteMax I lower intake and stock upper intake, and 1 1/2" long tube headers. Stock cam, stock SD SEFI.

I've got the parts to do this, I just need to giterdone.