You are going to have to actually loosen the holding brackets themselves. This can be accomplished by gripping the brackets with a pair of vice grips pliers and working the brackets back and forth in a circular motion around the long threaded bolt that is part of the windshield frame. Try to do this without #1 overgripping the bracket and crushing it, and #2 screwing up the threads on the bolt. Some of the gripping can be done on the wings of the bracket, again without destroying it. Penetrating oil will help. The rod tends to rust (bad) or freeze (normal) into the brackets after being cranked down for 30+ years. It's doable, just have to be patient. There won't be a whole lot of back-and-forth play in this bracket so don't expect to see it move a lot. As stated above, might be better to take out glass and change all rubber (and maybe glass) anyway. (Point to remember, replace glass as very last step in frame restoration.)
Bill