There are three previous threads here that are dedicated to this type of swap. You may want to go back through them and see what else has been posted on this. They are:
fj20-
T-18 -
CJ5 -swap-3373
T-18 -question-4765
will-
T-18 -bolt-direct-replacement-
T-150 -a-4602
I did a swap of a
T-15 to a T18A in my '81
CJ5 with the I6
AMC 258 i6 / 4.2l . My
T-18 came out of a V8 '76 J20 pickup and had a long input shaft. so some of the
T-18 's were behind V8's. The input shaft is key for your swap. Not only the length but the splines to. I had to put in a shorter input shaft with a different spline count which also necessitated a different clutch assembly. That part of the swap is pretty straightforward. Because of the length,width and height differences in the Transmissions, the drive shafts have to be changed, shortened on the rear and lengthened up front, and the skid plate/
Transmission mount had to be moved or customized to match up with the new
Transmission . It ran me about $900 to get it all done, doing most of the work myself. Knock off about $275 for the cost of the
Transmission and
Transfer Case on E-Bay and that's a rough estimate of what you are looking at. You also need to see if your
Transfer Case will bolt right on or if you need an adapter. I have to say that although it was not real easy, knowing what I know now after going through the Rubicon with it both with the
T-15 and
T-18 , I would do it again. I can't think of a better improvement for off-roading for the cost.
Hope this helps.
Tommy