Yeah those engines will be too underpowered for the montero, they are likely designed to run all day at 2 different RPM ranges one for idle and one for working. If these berry pickers drive themselves (not a tow behind deal) still would probably need a different cam and injection pump to make it streetable. Your best bet is finding a Pajero engine if you want fuel economy, otherwise you'll be looking at probably a loss.

1999 Montero SAS'd on Kings and stuff

1998 Montero trying to get a V8 Swapped