I've found just a couple of posts online in other Mitsubishi forums. Couldn't make sense of them for my needs though because they were for different vehicles, not Monteros.

I'm looking for any vehicles that engine blocks or heads will fit into these full sized models that I own...2001 3.5L XLS and 2003 3.8L Limited.

I'm also trying to confirm if any 3.5L long block motors can or can't be used to replace my 3.8L, and vice versa. I read the 3.8L block is an over-bored 3.5L block...not sure if accurate. If so, are the mounts, accessories, and ignition systems compatible?

The used motors for 3.8L are like $6,000 from a couple online sellers. That's insane. Would rather buy a whole Montero for $3,500 or less and strip it apart.

 

Thats a good one. I am in the EXACT same situation. The problem I see is that the intake of the 3.5 is a lot different than the 3.8. The 3.8 is drive-by-wire, whereas the 3.5 has a cable. I don't know what the result of swapping every thing over would be. I would like to know this myself. Weird. I also have a 2001 xls and a 2003 limited. I think I have a cracked head on the 3.8 and they are a fortune....if you can find one.