These 3.9 diesels came in Isuzu NPR trucks. Looks like a good canidate for a swap. Very similar to a 4BT but revs a bit more from what I have seen. There is also a company that makes a engine/trans adapter.
http://www.isuzudieselswapper.com/
Has one of these ever been swapped into a M truck? What are the drawbacks? Pros/cons vs 4BT?