Wow, now that's gonna require lots of elbow grease for sure!
I'd check some sites "for sale" sites for similar T-10 trannys. Some racer parts sites too. Can't even guess at $ yet a restorer would be interested.
Way too many modern 5-6 speed tranny options today have, I'll venture, killed the non-restorer crowd from any T-10, ST-10, M-21, M-22 etc., on using old school 4-speed trannys.