[ryanroo]

6.2 would be the easiest swap. i dont think you will see mid twenties with it, but i bet you could set it up to do well, assuming a lower 2wd truck. there are claims on the web of 4bt fullsize trucks getting mid twenties. i bet if you set it up specifically for mileage and not hot rodding or big power its doable with a 4bt. lifted 4x4s are going to get worse mileage. for the 6.2 i agree a 700r4 would likely do the trick. with a b series cummins you can pick from many. dodge, gm, ford etc. they are adaptable to most popular stuff.