Your gears will be on the plate in the engne bay. Between the Gen I and II, one is usually 4.625 and the other 4.636. I never remember which without looking at mine.

As for the rear disks, how is your rear sprung now on the 89? If coils, best overall route is to swap an SR locking axle in. No real "upgrades" to the stock drum axle. Some overseas market I think had rear disk possibilites.

Big Truck: 00, 3.5, Endeavor, 5-Spd drive line in hand!
Little Truck: 87, 2.6T I/C, MT, LSDs, Tonneau Top
Her Truck: 03, 3.8, 20th Anniv, 65k
Daughter's: 06 Eclipse, Keeping it Mitsu!
FSMs: MitsubishiLinks.com