That's some serious rust on the windshield header. Interesting that it's the only real bad spot.
Since you have the other cab to get a patch panel from, that wouldn't be a bad repair. Carefully grind away the inner panel on the rusted cab, leaving as much as the outer layer of the pinch weld as you can, and do the opposite on the cab with the good header. Clean up any remaining rust in the roof panel (media blast or whatever) and weld in the patch panel. Should be able to do it so the repair is invisible.
