The 68rfe is a solid trans when used the way it was designed to be used, Theres a member on here with over 550,000 miles on a stock 68, hot rodding and racing will kill them quick though. Another thing to look for is service history, make sure it was well maintained, if it has more than 67,500miles then make sure the CCV filter was changed, EGR, EGR cooler, EGR crossover tube, map sensor, DPF, NOX filter were all cleaned. Another thing to stay away from is a previously tuned or deleted truck, most of them will be driven pretty hard, so the trans would be abused. The 13+ trucks will get better mpg in 100% stock form. That's about all I can think of right now, let us know if you have any questions.