I seem to remember the 3.5L oil pan being a rear sump just like the SWB 3.0L. May not need to mess with the oiling system at all as long as the donor 3.5L comes from a montero.

Motor mounts will be fairly easy, but the extra width of the DOHC heads might be the hard part. I wonder how much narrower the SOHC 3.5L is than the DOHC version?

1994 3.0L 4x4 pickup.
4.62 gears (4.22 stock), 33x10.5 BFG MTs. LSD rear. Five speed trans. 172K
Previously owned Mitsubishis:
1986 2.6L 4x4 Mighty Max, 1985 2.6L 4x4 D50
1987 2.6L Raider, 1989 3.0L Raider
1995 Montero SR