The soft rebuild parts are out there. The hard parts are the problem. If your case, shafting, hydraulic control housings, and torque converter are all repairable then an overhaul is no problem.
A lot of the cases get cracked when folks get stuck in rainy or winter environments and they start going back & forth with reverse and drive. The case can crack at a weak point inside. This is common on all the cast iron case Borg Warner designs. As was also mentioned, certain surfaces can wear below specified tolerances causing a search for hard parts. Torque converters on the old ones are hard to find parts for. Later ones can be substituted but shafting has to be modified to accept them. The air cooled design started to phase out in 56 & 57 when they started adding oil coolers. In 58 the dual range cruise-o-matic came out and that put the old single range transmissions on the chopping block. By the end of 1959, they were done.
If I HAD to replace one, I'd spend the bucks and get an AOD conversion but that's just my opinion. There are still parts out there now. It just takes a while to locate them and you have to know where to look and what to look for.