I may be wrong, but I believe the Federal DOT mandates that windshields are laminated safety glass.
I understand your rust problem. I had a 68 Impala convert. In 1970 I replaced the front floors. When I junked it in 1974 the rust was so bad that the dash board had rusted away from the cowl frame.
If your windshield is still good, leave about 2-3" of roof along the top of the windshield and box it in and you should have all the support you will need.
Assuming you are only going to weld the rear doors shut, if you add support to the B pillars at the floor you shouldn't need to add support on the doors. Use a filler piece in the door gaps when you weld. Leave the hinges in they will give you good alignment and will keep them in place when welding.