It's the same engine/transmission bolt patterns, if memory serves me right. The transfer case mounting pattern and output shaft are totally different, so you'll need to use the D300 transfer case that came factory behind the T-4. I wouldn't swap out a T-150 for a T-4. The T-4,T-5, and SR-4 4 speeds are light duty. They'll break if you look at them wrong. They all have better ratios than the 3 speed so swapping them in is tempting, but they won't hold up.
The granny-low T-18 is a heavy duty unit. I has great stamina and will make a Jeep much more capable in slow speed wheeling conditions. But in my opinion, it sucks for everything else. It works like a 3 speed in normal driving conditions, shifts like a schoolbus transmission and weighs a ton. I prefer the lighter duty T-176/D300 used in 80+ CJs for a simple swap. It is considered a "medium duty" unit like the T-150 that you have now, it's much stronger than the T4,T5, and SR4 but also much weaker than the T18.