Dump truck trans for the win! Just need on-board air also for shifting. RTO14908LL 10 speeds of fun. Overdrive plus a 14.56 first gear in Low Low. And 3 reverse speeds for when you gotta go. Only weighs 690 lbs but it's rated for 1450 lb ft.
You just gotta go bigger to get what you want. I would seriously consider one of these for my super duty some day when the slush box dies. I'd just have to find one in a salvage yard behind a 7.3L to get the correct sae bell housing and then do a divorce t-case.