If you are looking at the V-8 Magnum for excellent mileage, forget it, the very best I have seen is @ 18, not surprisingly, it was a non-over drive automatic 318 in a '92 Dakota, 3.21 gears, with a hollow cat.
Overdrive, even manual, really seems to affect the mileage on them.
A TBI or carb does better, mileage wise.
Mark.