I feel your pain. I have a 57 small window with a severely damaged and rusted cab ( mostly fixed now). I found lots of used cabs in great condition. The problem was they were all on good trucks! So, since I was not willing to replace the whole truck with a better one ( it was my grandfathers farm truck) I started to repair the cab in my spare time.That was about 7 years ago.
If I bought a new cab I'd be done and driving it by now, however, would it be the same truck? I'm not sure at what point a restoration becomes a replacement. I've had many discussions with friends on this subject over many beers. Some think the soul of the vehicle will be transferred to the new cab with the VIN plate -I'm not so certain. Anyway, I couldn't afford a new cab at the time, so here I am.
Hope that was ambiguous enough for you!