I am putting some plans to paper for my resto / custom 67. I would really love to pull out the side "vent" windows and frames and replace this with a solid piece of glass like the later models.

Does anyone have any sources for custom glass like that? Maybe lexan is the way to go?

check your local laws, Lexan may not be legal. You might want to purchase a Street Rodder mag of some sort on the newsstand to find a source.