I have a 1989 Ford F-350 crewcab dually that has been lowered with a Bell Tech drop kit, 6in. in front, 8in. in rear. It has a ground effects kit, Blowjacks air suspension kit at all 4 points. It has a rebuilt 460ci with trick flow heads/intake and a Holley 1100 carb. Modified exhaust by Cat Back. Other than paint and interior, most work is done, but I want to get rid of the stock AOE tranny and rear gears and go with something more fuel, highway and high end apeed friendly. Looking for some suggestions that are an easy swap. Thanks!