Dunno if this helps or not, but when I first bought my '65 C10, the bottom threaded part on the bottom of the vent window frame was broken, but still there.
I took it all out and cleaned up the vent window frame, then had the threaded part welded back onto the bottom of the frame by a professional.
The trick is to know which way the piece fits on, the first time they welded it on backwards, because we had no idea until I went to install it.
I do not remember if I took the glass out that time or not... it may have fell out because the adhesive was rotten. I dunno.
Good luck,
-W
