The input and output shafts of the two transmissions have the same spline counts as one another. I’m not sure about the length of the input shafts but if you have both transmissions it will be easy to measure. If a transmission bolts to a CJ D20 xcase it will bolt to a 1980 Scout D300. The key is if you find the Scout D300 you need to get the bull gear off of the T18/19 it was originally mated to.
Also I’m not sure about the transmission bell housing opening and bolt pattern, I don’t think the bolt patterns are the same between the T15 & T150 but don’t know for sure. You may need to re-drill and tap the bell housing to match the transmission pattern assuming the transmission fits properly in the hole; if not try to pick up a Jeep T15 bell housing at a junk yard, Craig’s list, or ebay as that would be he cheapest way to go. A stock CJ bell housing from either transmission will bolt to an AMC I6 or V8 of the same vintage. However, the 1972 CJ clutch system was a little different than the 1976 & up at least in how the clutch was activated. The 72 used a cable that was prone to failure as the ball on the cable would strip off or the bracket holding the cable to the firewall would tear away from the firewall leaving you without a clutch in the wilds of Montana a hundred miles from anywhere. (Sorry, personal experience there) The cable was later replaced with a rod. I only mention this to let you know but if the fork system in the bell housing is the same with either system then you should be good. I would think the guts of the clutch should be the same. (Maybe you have swap out the throw out bearing fork so the connection is correct?)
