The lower 1st gear
The 700r4 or 4L60e w/their lower 1st gear would be the choice since you have 3.42 diff. A towing converter w/lock & lower the stall speed the better IMHO because of the 6.5's tq.
NOTE: If going w/700 r 4 make sure you get one w/case manufactured after 1987, otherwise you'll have nightmares because of failures. Trans can be built almost bullet proof these days, and with such a build and proper cooling will far outlast anything stock.
B&M's SuperCooler w/fan & thermostat control would top off project nicely.
99 GMC SUB, 2500, 4x4, 6.5 TD (F) #H32 enhanced blk, Phaser timing set, B&D IC, 3" ATS exh, K&N w/prefilter, 4WD NVG 246, 4L80E kevlar Transgo off road/tow mods/B&M supercooler, 14 FF MagTec w/locker 3.42:1, 9.25" IFS frt/diff 3.42:1 w/ARB locker, AutoMeter Boost & pre-turbo pyro meter, 12,000 lb hydro winch, Warn classic bumper w/brushguard & diesel scoop, Max E-torq Stage 3, hi pop inj, oil bypass sys, on board air comp for front ARB locker & tire inflation, DynaView driving/auto-cornering lts, DZ nerfs, A/T 285/75/16 SilentArmor 3750 lb rated on 3750 lb rated Alcoa, 3 IP drivers 2 r spares.