There are three main engine families in the 6Gx series.
The 12V and the DOHC 3.0 are the early style engine.
The second gen is the 3.5 DOHC.
The third gen are all the 24V SOHC engines, these have the oil filter on the drivers side of the engine.
All of the engines have the same bellhousing and flywheel bolt pattern.
There are many detail changes as to the bosses on the side of the blocks but they all can be interchanged with a mix and match of brackets.
As long as a transmission came off a V6 it will go on another. In the 2nd gen Montero the trans crossmember is reversible for the different length transmissions. The driveshafts are different length for each trans/ transfer combination.

I do want to keep the transfer or at least the full time ability when I swap, I do not know if all the parts are in country for this.