Phoenix is supposed to be good smart transmission people, but I'm not sure I would trust them on this one.
If GM were building this truck it would have either a 4L80e or an Allison, definitely not the 4L60 or 700r4.
I have a 350 powered 4200 pound truck with a 700r4 that is failing I'm sure at least in part because of the load its been asked to move.
