1) T-case shifters can be cable operated (WJ Jeeps), Remote linkage (TJ's, MJ's, XJ's, ZJ's KJ's) or on a bracket Bolted directly to the trans/T-case (YJ's, Dodge Trucks)
2) Drivelines should be not much of a problem, the rear shafts use the same size u-joints, and a driveshaft shop should be able to make a front or rear shaft pretty easily.
3) On models with mechanical speedos, the thread size is the same on the speedo adapter, but the tip on the cable is square on the Mopar, vs round with a tang on the Mitsu.
The connection at the speedo head is the same as an XJ Jeep, most likely the cable from one would work, or you can match one up at NAPA (info above based on my friends '86 Montero and YJ parts that I have).
I checked my friend's 1986 Factory service Manual and it did come with a 904 Automatic, if it had had the Auto Trans option!
I wonder how hard a 3.9l V6 Mopar would be to install in a Montero using a 3 speed auto by swapping the trans case and using the Mitsu tail housing and output shaft and original t-case? Maybe the rear output parts are the same as a CJ-7 3 speed?