The 87 Starion rear diff fits the V6 Montero / V6 Raider front diff if you use the the 4 cyl CV joints. You also need to mod the cross member to get the pinion mount to fit.

Three versions of the front LSD swap were figured out.

The first was by a guy in AZ... Forget his name ( Mitsutech). That was the early Starion LSD into a 4 cyl diff.

The second appears to have been done by Kary, that was done by using the 87 Starion LSD and 4 cyl axles.

The third version was to use the 88-89 Starion LSD with the V6 CV joints. Some guy from California named Kevin worked that swap out.

It sounds like you have the swap Kary worked out.

I have had everyone of these parts for every version of this swap on my workbench at one time or another.

Someone would have had to swap the diff housing from a V6 into your 4 cyl... Or you have a V6 Chassis. I'm assuming you have leaf spring in the back?