Ouch that is a nasty looking fork! I see why you needed to replace the internals.. what did the second gear look like? I only had issues popping out when shifting within 2k or so of redline, so probably got lucky and installed the kit before any major damage was done.

I think the stock spring was too weak to begin with, and after 20-30 years of use becomes even worse. The Factory Pro design is definitely worth the upgrade, but if it's popping out of gear that early it may need the internals replaced as well. If it were mine I would install the shift kit first, but be aware of the possibility of needing to tear down the engine if the shift kit upgrade doesn't fix the problem.

 

I didn't tear the gear stack down but it wobbled really bad. The new gear set holds all the gears but its really noisy so it has to come apart again