T18 is a heavy duty transmission, and will not bolt up to the Dana 300 Tcase without $$$ adapters. The T18 is a stronger transmission, but the T176 is easier to drive (T18 has a non-syncronized 1st gear).
For the non-monster truck, the T176 is the cheapest, easiest transmission to upgrade from a SR4, T4, or T5.
You will have to either buy new driveshafts or have yours shortened/lengthened, and you'll need transmission, tcase, adapter between them (if you're buying a set, it's there), bellhousing, and tunnel cover with shifter boots. Your skidplate will work, but you'll have to move it up a bit. There are different sets of bolt holes already in the frame to allow it to be put in different places for different transmissions.
I'd get the T176.
85 CJ7 258, borla header, Team Rush, MC2100 ,optima batt, Warn 8274, agr super box 2 & pump, T5 w/ B&M shifter, centerforce 2, superior 1 piece axles, warn hubs 2.5 in lift, 31" BFG MT
85 CJ7 258, A/C, T176, MC2100, Team Rush, Warn XD8000i, superwinch hubs, 3in lift 31" BFG AT/KO