welcome to the forum.
before you commit to repair do a feasibility work up on it. poke everywhere and get a list going on what you need to repair. these things leaked from the vents onto the floor so everything below that vent in front of the window outside is subject to a thorough investigation. leaves and pine needles would collect and plug the drain at the bottom of the cowls, on each side) and the rot would begin. on mine I thought it would be a fixer and started cutting out the rust. turned out to be way more than anticipated and I have replaced the firewall, both cowls, both front hinge pillars-inners and outers, and both rear pillars, the floor, both of the front the cab mounts, both of the cab steps and rocker panels-along with the bracing behind that, the area above the windshield (part of the roof panel if you are buying one. a good spot for mouse nests and other debris carted in by pests-subject to rust because of build up), both of the rear cab corners-inner and outer, the rear cab mounts, some of the winshield lower pinch weld-hidden by the rubber strip until taken apart, all the door hinges and have not even got started on the doors yet. they will need both lower skins. of course there is disassembly time, storage space, sandblasting, sheet metal parts, welding equipment and supplies, primers etc etc. plus you need to take some good measurements or have an assembly manual for the proper specs, before you take it all apart. I would also install some bracing to ensure things don't move when the parts are cut out. if I was to do it all again I would look seriously at a whole new cab from brothers or equivalent. since I have to take time off a sideline job to do work on my truck, it would have been more economical to have worked that time and just bought the cab-which comes with doors, hinges etc.
not trying to discourage you, just a heads up before you start. take lots of pics and measurement recordings before you start, if you plan on a redo of the old cab, that way when it has sat idle for a month you can go back and see what you are supposed to look like when done. if you plan on another used cab look for a big truck, like a 1 ton or larger, they are higher off the ground plus maybe seen less time on the road. the cab is the same but the fenders etc are different. just as a heads up also, the chev and the gmc may use a different length frame. the chev being the longer because they had a longer filler behind the bumper. I have a 57 gmc and a 58 chev. the 58 frame is longer ahead of the front spring mounts. they look the same otherwise.
good luck. take some pics and post them up. we all like to see that.