[D13]

Laminated safety glass is made from 3 layers - inner glass, outer glass, and the PVB inner. The PVB is pretty much a standard thickness. The glass layers are the key. The glass sheet is floated, cut, heated and bent, then the layers are assembled.

The glass thickness is controlled at the float. Only a few standard thicknesses are floated, and then the manufacturer decides which thicknesses they will use for the application. In the last 10 years the available thicknesses have condensed and gotten thinner. We used to do a 7mm thick windshield, now the thickest anyone makes is 5.4mm, and 5.0mm is the standard. So the glass fab probably cannot afford a custom thickness run, so you gets what they give.

As far as these specific trucks, I think they were 'light weighted' in the 1980-82 time frame and that may have included thinner windshields to go with the thinner door glass. Either way, the only way to get an 'original' thickness glass is to get an 'original' used windshield. And most of those have been replaced at some point - so it's kind of a crap shoot.