[Bruce Edwards]

Yes most are bolted on to the frame from underneath. You can also try taking the side screws loose and see what happens. Just do not take them out to far as they are very short. If the frame is under pressure the screws will feel like they are draging and you can see the outer frame rail pull away from the windshield frame.
The fix can be easy or hard depending on how it was installed. Most are bolted to an upright. If it has shims between the upright and frame rail just take some out or add some as the case may be. If it is rubbing on the body you may have to file the body opening. I have seen some cars with the frame so tight you would think two guys were trying to pull it apart to get it in. When you take one out under that much perssure be sure you do not let it get away and scatch the top of the cowl.
If the frame is bolted straight to the supports depending on how they are made you may be able to bend them a bit to get some pressure off.
Remember also the outside frame rails on the windshield are brass and will bend if forced. What you want is a windsheild that slides into place with little or no drag and is straight to the supports so as it is tighted into place everything stays the same with no pressure pulling on the frame rails for the windshield.
Sounds harder than it is.