Our resident Kiwi nailed one of the mpg issues, the Effies have the aerodynamics of a brick!!! However, a well planned build could certainly make the mpg tolerable.... My first choice for power would be a 5.0 with a 5 speed... Lots of old Mustang donors out there. Or, step up to the newer modular 4.6L engine with either the 5 speed or the AOD if you want to be "shiftless"... Hard to improve the aero without some extensive mods, but during the build process, a bit of lowering and clean up on the underneath would help....Try not to have a lot of brackets, exhaust pipes, etc hanging down to break up the air flow...no big deal, but everything helps. Also, a rolled pan on the front will help get the air off the bottom of the front end... But all that frontal area and big tall cab.....mpg just wasn't a consideration when these trucks were built.....