Is your 67-76 A-body windshield leaking ?

If the rubber had a weak seal to the body, which they did because they were not glued to the body they just leaned on and touched it, the water would pass between and lay in the valley. If the valley was full of water it could either drain to the cowl, as designed, or overflow into the car because of plugged drains. The "plugging" was result of the sealant used in the attachment of the cowl to the shell, not the windshield rubber to the body. Some cars actually were not plugged! When you expose yours, you will see the material is a foam type stuff vs black window seal that was used by later installers of glass. This is what I have found.