Fred, I've been thinking about your transmisson for a couple hours, off and on. Trying to think of the best way to go, That T-18 would be cool but, for your intended use a T-176 would be better (also means a Dana 300 ($). My trail CJ 5 has a 6.32-1 T-18, 4.30 gears and 36" tires. It had a T-176 before, and I wheeled it hard without problems (just had to feather the clutch more)I wanted the lower first for rocks and stuff.
So, because of the added cost of buying a 300 TC. I would stay with the T-150, I can't imagine needing 4 wheel low and the 6.32-1 for your use. Your jeep when finished wouldn't pull the tractor or the dually pickup out of a stuck anyway,with either tranny, its just doesn't have enough weight.
On edit, my DD has a 258/T-4,Dana 300/ 2.72 gears, and 31" tires. I think it would pull a loaded hay (3 or 4000#) wagon in 1st gear in 4 wheel low without a problem. Stopping would be another matter.
1957 WILLYS pickup,
1973 J 4000,
1978 CJ7 DD.
1979 CJ7 360, TH400/Quadratrac.
1980 CJ5 trail Jeep.
1989 YJ the CJ to YJ.