I love keeping things within the group, or even locally, but if you are grasping at straws, I think there is a place called West Texas Offroad that rebuilds the boxes.
https://www.westtexasoffroad.com/Things have changes since I last heard any correspondence about them, but worth a shot. Also, if you plan to offroad this rig, you may want to at least have the discussion about drilling and tapping this box for Hydro-assist. Way better to do it now! Even if you don't use it, the ports can be capped to be used at a later date.
1973 Bronco, 351 SEFI, Locked, discs, 35's ZF-5spd and Atlas 4spd. 235:1 Crawl Ratio. It may be ugly, but it's slow.
http://www.ucora.org