68 SS Door glass or lexan ? your idea ?

Keep in mind that even treated Lexan will scratch eventually & depending on how accurate you want to be, it just doesn't look like glass. The original cars were known for breaking the side glass out simply due to vibration & useage, so I'm not sure you want to go that route either. If it's a dual-purpose car, I'd stay with the stock glass. If it's a dedicated racecar & you don't mind the very minor imperfection lexan will give, then I say go with that.
I will add that years ago I saw one of "DeFrank & Cohen's" S/S 'cudas & the owner added extra lining in the channels to insure the Corning glass didn't rattle itself apart. That may be another route you'd want to look into.......