You need to find a good shop with good transmission mechanics-not a parts jerking overhaul mill! On one the problems with the proliferation of the sledgeomatic is that not only was the quality of operators degraded, the quality of experience in people who could "overhaul" a transmission was degraded. Heck, blast it with the steam cleaner, slap in a new valve pack or tighten up the bands, squirt it with some Krylon and you've got an "overhauled" automatic transmission...
How much weight are you putting on the truck with that Gooseneck trailer? The Getrag is not a bad transmission but it does not tolerate (from my understanding-my Dad didn't hurt the one in the '93 D-250 and I've still got her-she's not running right now but that's a different sad story.) being overloaded well. With the overfill you were doing-if the fluid was being changed on schedule-it should have stayed fairly cool but I don't know enough other than to say anything more than you need someone really well qualified to make an estimation...