That's been posted before, but I can't find any proof of it.
Attached is every federal safety standard since they started in 1968. The only one I can find that pertains to windows and glass is what they can be made of.
http://www.nhtsa.gov/cars/rules/import/FMVSS/#SN101
The small windows were still used on the larger (C-40+ trucks after 1967).
It's been discussed on here before and it's believed it was a cost cutting measure. Why have two cabs when you only need one. The original designer of the truck did an article years ago where he stated he had to "fight" to get GM to do the small window.