351 Fuel Injected. Or a 460 Fuel Injected made between the years of 1988 - 1989. With ur original plan I don't think that Dave put the 35" Swampers in to the gas miliage equasion, I think with that setup you'll get 6 MPG if ur lucky, lock it up in 4X4 3-4 MPG drop in 4 low probably less than a gallon a mile. Fuel Injected engines are usually far more echinomical.