I cannot speak for Monteros specifically but I have in fact swapped a 12v 6g72 for a 24v 6g72 on two separate occasions in the past. These were both on Dodge Stealths. Taking from that experience, you will need the main wiring harness and the ECM. For those swaps, there were other things that also had to be reworked. The wiring to the alternator was not on the main harness and completely different. This had to be repinned. The MFI relay was different. The cruise control did not swap over correctly. You may encounter other body wiring harness that do not swap over correctly like the dash and speedo/tach harnesses.

Your best bet is to buy the entire truck so you will have everything you need to complete the swap. Then give the body back to the junkyard to crush (if they agree to such a thing).