A 289 Ford V-8 would probably??? give an edge too. I forgot to mention I used 15" tires off of a 84 ford half ton. If you could gear it right and use 14"ers, it may help aerodynamics, mileage, etc.

Good advise Albuq. I used to get 13 MPG with my 3/4 ton with stock low gears and flathead, I think they were 4.11 or something like that.