I know the 92-93 ys1 cases are bigger and the trannys have bigger splines. But can the 5th gear fit into the 89-91 b16 y1 LSD trannys? Or does it have to be the 90/91 ls tranny only.
The part #s between the years are different. The part #s being different isn’t always a guarantee that the parts aren’t the same but it’s usually an indication that SOMETHING is different. Since it’s pretty commonly known that 90-91 and 92-01 internals aren’t interchangeable as a whole I wouldn’t be surprised if the two different 5th gear assemblies were not interchangeable.
I’m just speculating. I have no experience with the early (90-91) transmissions but if I were trying to source parts and was in your situation I’d be looking for parts from a 90-91 transmission.
If only .J. was still around here…