My bet is that the GTO kits fill the windows with fiberglass. Welding them up should be no big deal though. Remove the windows and weld in some 20 gauge steal sheet. You will have to fab an edge for the weather striping to mount to, but again no biggie. The work comes in smoothing it out. Sand, fill, sand fill. If you're doing that you may want to shave the drip rails too. Terry O/Blueoval has some pics and advice on that on his page.