A 15x8 wheel with 2" BS will position the rim edge similar to a 15x10 with 4" BS, logically. You shouldn't have problems with 31s rubbing anything with the exception of the front bumper rubber edges, possibly.

Flares to cover the exposed tire will be custom, Bushwacker does not make any of their typical flares for the Montero.

1987 Raider - Roxy
1988 Mighty Max 2.6L Turbo - Pearl
1997 Mountaineer V8 - Freddy
2000 Excursion V10 - Freya