Not that there is anything wrong with the T56, but I'd just look at the NV4500.
Car tranny in a vehicle probably 1500+ pounds heavier is going to be like the 10 bolts in the same two vehicles IMO. Awesome in one, destroyed by the other.
I know the later 4500's didn't have the super low first gear of the early ones, so the gear spacing (you'd hope) would be improved quite a bit over the early ones, which pretty much matched up to the 465 gear spacing, with 5th just being OD.
Besides the fact, 4500's are fairly regularly retrofitted to these trucks in particular, so you know it can be done without too much effort.