i have an "89, and found the same problem- was told the case was "too thin" to cut for the gears.
it's actually "too short", using narrower gears and shorter gear shafts, with a different end casting.
i used one from a '93(i believe '92-'94 works).
i discovered the true difference when i used my original as disassembly practice for an overhaul.
i am now using a "hybrid" case- overhauled my original, clearanced for the gear,+ the '93 shaft and end case.
got my gearset from trailtough.
richard