I worked on your trans this afternoon. There are a few concerns that you need to address before I go any further with it.
This trans has been open before. Whoever opened it sealed it back with black RTV. That's a big no-no in tranny land because it hardens (rubbery hard) inside the case (when the case halves are put together it squeezes some on the inside and outside) and eventually the rubbery pieces make their way up into the gear shafts which plug the oiling holes in the shafts.
I noticed when opening the trans that it was sealed with RTV so I knew that there would be some extra cleaning I'd have to do to it in order for it to be 100%. I also noted that the trans was stuck in gear, I wasn't able to put it into neutral prior to opening the case, but it wasn't any real big deal because it happens sometimes. It's just easier to disassemble when it's in neutral. However, why it was stuck in gear is another story.
In this picture you can see where the RTV was squeezed out on the inside of the case. You can also see RTV pieces near the bearing race in the top half of the case.
When I disassembled the main gear shaft I found that RTV had indeed worked it's way up into the shaft and had completely plugged the oiling holes for 3rd gear, 3/4 synchro and blocking rings and 4th gear. The bearing also has RTV stuck in it, however it doesn't look to be damaged and can be cleaned out with no problem.
The 3/4 synchro is toast. You also have all brass blocking rings so this trans is originally out of an 89 SHO. The 3/4 synchro has burrs on it from not being oiled properly so it can't be reused. This is also why the trans was stuck in gear. The lack of oiling has also caused some damage to the blocking rings as well.
RTV:
This actually has to be one of the largest masses I've ever removed from a gear shaft in my years of rebuilding.
One of the rivets that holds the ring gear on is damaged. I'm not sure what caused this damage, I don't see in a large pieces of metal anywhere inside. It would take a LOT to do this kind of damage to a rivet, they're very hard. This won't cause any problems, but I wanted you to be aware of it.