If you replace the top of the cab, you would need to cut the windshield pillars where the lead filler is and then cut the cab just above the pinch weld in the back so that you can get to the pinch weld which holds the top of the cab on. That would entail cutting out what will seem like 200 spotwelds. Its not that many, but it will seem like it, Once you do that, there is no easy way to reweld all of those spotwelds due to the interference of the 3 inner cab panels.
You other option is to cut the top off above the rear glass, but that would entail a very long butt weld which is difficult to do without warping metal in a very visible area.
