Pilkington Glass sells the date coded, heavy thickness glass just like the original pieces. I used them for the windshield and door glass on the black Firebird. The installer commented that he hadn't come across replacement glass that heavy in decades. I guess most modern replacement glass is thinner now.
http://www.pilkingtonclassics.com/
They ship in heavy, wooden crates the size of coffins! (very expensive shipping costs - be forewarend)