How do I seal my windshield on 67 Barracuda?

Not close to an expert, but to seal a leak in an existing gasket, I first try "flowable silicone" in a small tube and "butyl rubber caulk" in a caulk gun. I recall the later comes in black. I think butyl rubber is the stuff used to attach windshields in modern cars.

When installing a windshield, I suspect you could only caulk the inner surface since one usually puts a rope around the perimeter and pulls that outward to flip the gasket lip over the glass. Anything you pre-filled in there would make a mess.