Hello, we install glass and windshields on vintage mopars at my shop.
What happened probably is they tried to use urethane sealer with the windshield gasket and this WILL NOT WORK.
You are going to have to pull the glass back out and probably get another gasket. You will need to reinstall the glass with a new gasket and before the lock strip in put into place you need to seal the gasket to the glass and also seal the gasket to the body with Auto Bedding and Glazing Compound, 3M part number 08509. This is basically liquid butyl rubber and is designed to stay flexible. The urethane dries super hard and will not properly seal to this rubber gasket.
I see this on a regular basis with these glass install companies as they mainly work on late model cars and are pretty clueless on antiques.