I have installed dozens of front and rear windshields on older cars like ours. I have NEVER used any sealant at all, and I have never had a leak. Make sure everything is clean, pop the glass into the gasket, put water into a spray bottle and add several drops of dish washing soap to it, spray the groove that goes over the metal lip around the windshield opening liberally with the soapy water, insert heavy twine in the gasket groove you just squirted the soapy water into, squirt the metal lip that the gasket will slip over lightly with the soapy water, place the windshield with the gasket installed against the opening and finally pull the rope/twine slowly to pull the gasket over the metal lip. This is a high-level overview. There are lots of videos on YouTube that show the process. Like I said, I have never had a leak. Did they use a sealant at the factory?