My bud had a 93 with a cummins, he got 24-25 MPG with it. My boss has a 97 cummins and he gets 22 mpg with it. Cummins would be my vote. or a Ford to tow the ford...
My dad has a '05 dodge 2500 diesel and that thing is a monster! It has so much power and torque; it gets about 20mpg right now. Once he's done breaking that engine in (owner's manual said in about 10,000 miles it will be done breaking in) it said it would get a little better gas mileage.
My dad raced my fiance (has a 95 GT) and whooped him like he was in reverse!
I have a '97 dodge ram and it gets roughly 17-19 mpg right now. I haven't towed my mustang yet, but the gas mileage goes way down then I'm sure lol.