When Oklahoma had inspections, the only glass you had to have that was glass was the windshield. The other windows could be Lexan.
You wouldn't want to use Plexiglass, it's more scratch resistant than Lexan, but it breaks into big pieces. Percy's Speed Glass has a special coating for their Lexan that makes it scratch resistant. I saw it at WFC8, they had a piece that was half coated with some steel wool and the uncoated side scratched easily, whereas the coated side did not.
I've seen guys put Lexan in their early carsm they just removed the glass and put the Lexan in the old frame. Problem wasm the Lexan was thinner. If you were to maybe take a rear track and put it at the front of the door (from the opposite side of the car), and took some extra window channels and welded them to the existing channels, benind them as required, then re-chroming them, that might work, but the side glass has a slight curve to it, so you'd need to find a donor glass that had the same curve, but was bigger. I've got some Fox body glass, but I think it may have more curve to it.
It's be WAY custom, but I think the best way would be to cut away part of the roof and the A pillar and attach it to the door, more like a modern car. You could then make the glass fit more flush along with the quarter windows (unless you have a fastback). The rear of the window frame could fit on the inside of the window, so when the door was open, there wouldn't appear to be a rear part of the frame.