8.50" & 9.25" GM IFS diff
There are three different GM ifs diff and I'm thinking the 8.50" (for 1500) and 9.25" (for 2500/3500)have the same mounting locations.
Suggest you check you RPO codes to determine exactly which IFS you have, doubtful a 1500 would come w/8600 lb 9.25" frt/ifs even in the diesel. An 8600 lb ifs diff has 14 bolts clamping the case halves together.
I've seen GM IFS diffs on pirate 4x4, and various search engines, and most bone yards what makes it tough is the gear selection 1999 8600 lb IFS diff came in only 4.10 or 3.73 period.
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.