I picked up the transmission today. They built the unit with the early low gear ratio in the later style wrapper. Fresh synchros, new bearings throughout. Anything it needed it got during the rebuild. We tossed my core on the cart and he asked if I wanted to see inside. Hell yes, I do! We rolled it into the shop and he pulled the shift tower off the main housing. It was a little anti-climactic. No major carnage like chewed-up gears. The input gear had some knicks in it, but the issue really came in with the loose input shaft/gear and how it interfaces with the direct drive (4th gear) synchro. Makes total sense when you see it apart, but the 4th gear synchro is first in line from the input. You could see as the input shaft wobbled in any direction it was putting tension on the synchro. Which explains why the thing was popping out of fourth when I let off the throttle. The change of torque on the line would cause the input to wobble and the synchro would let go.
There was more damage from the garbage getting run through it, but I didn't stick around to witness the rest of the autopsy.