The window channels that you refer to are held in by one more screw at the very top. You can see where these are located by looking at this view of the channel.
https://www.flickr.com/photos/advance-design_parts_co/9479860265/in/set-72157631868452754
I don't know how much work is required with the Altman system but I am guessing you will want the window out for access and to prevent grinding around it. It is only held in with 4 small clutch head screws but soak them good in penetrant because they can be tough to remove.
https://www.flickr.com/photos/advance-design_parts_co/12646902905/in/set-72157641244124604

1950 Chevrolet 3/4-Ton 3600
More pictures on Flickr