Got the mainshaft cleaned and reassembled today.
The synchros, snap rings, and shift collars all got replaced.
Third gear being reinstalled:
The whole assembly back together:
And tucked into a clean garbage bag to protect it from dust:
Reassembly was pretty easy with this one. For the shift hubs I took a small deadblow hammer and tapped downward around the edge of the hub until it seemed to seat.