Disassemble of old cab is a good idea. It will tell you if your plan will work. It also does not destroy anything if you decide to patch using the old cab. I would think that patch is better for me, but I have not examined the whole picture like you can having the parts right there. Get a good quality spot weld cutter/drill. You will develop a technique as you go and then get better and faster.
A possible risk I see is with the wrap around windshield. They are very hard to install. I've done a few 55-59. There is a dimensional tolerance issue because of tolerance of the glass and tolerances of the opening. Sometimes causes big headaches. If your effort involves the opening, it is possible that you will change the size and shape of the opening. Doesn't take much for a wrap around style. 63 is the last year for those. The 64 and up flat windshield is way easier. You could get all done and windshield does not fit. Are you getting a new windshield or using the old? I realize you are changing cabs and may or may not be matching windshield glass to it's mate. The point is that you do not want to end up with a windshield opening that is beyond the factory tolerances. The opening is impossible to measure so you won't know. If at least one half of the windshield pinch weld stays stationary, you should be OK. Just try to figure out how the factory positioned the pieces before welding. Maybe they have location features built in. If they used jig fixtures, that's a problem. That opening HAD TO be within factory tolerances so rubber and windshield would go in every time, no leaks. A big windshield and a small opening and vice versa.
