I agree with the guys above,and if it's a 5 window cab,you can still buy the corner glasses new.
If you have the original glasses it's the best possible templates you can get.
Some of the aftermarket stuff leaves a little to be desired.....
Any good glass shop will be able to do this.

1950 Chevy Advance Design 3100 in Scotland
In the Stovebolt Gallery
More pix on Flickr.
I've definately got this truck thing in my blood ... my DNA sequence has torque settings
"Of all the small nations of this earth,perhaps only the ancient Greeks surpass the Scots in their contribution to mankind" Winston Churchill.