I don't know a whole lot about the transmissions I just know the 68rfe is the diesel transmission and the 66rfe is basically the 68rfe gearset in a 545rfe case and is for the hemi n the 2500's. That gearset from the 68rfe or 66rfe is a much better gearset than the one in the 545rfe(ratios are spread more evenly, lower first gear, and taller final gear), and it is supposedly stronger which should be obvious since it's from a 2500 ram not a 1500 ram. I'd be really interested in more info on the transmission work, and cost, as I'd be interested in it myself just for the better ratios. Thanks and good luck with the truck!
Breathing in and out is good enough for me.