I just rebuilt a 1980 j-20 box. It's all easy except if the loose ball bearings fall out of the piston when you disassemble it, which of course mine did. The instructions say be careful and don't let them fall out, but then they don't tell you how to get them back in if they do indeed fall out. I got them in, and it seems the piston moves freely from lock to lock, but we'll have to see once I install it if I did it correctly.
So, what goatman said, run it first, then rebuild if needed. Also, first make sure the hydraulic fittings and the steering shaft splines are the same. I think it's iffy on an 80, but either way, those parts can be swapped to work. All my fittings appear to be correct. _nicko_