Okay, then you are supposed to pry the gasket into the metal channel with the glass in it? You write that you don't know why it helps but I wonder if the top corners are too sharp of a corner? The last glass I put in was a 65 Barracuda. Old glass, old gasket. I set the gasket in the channel after sealing the entire perimeter, set the glass in the bottom and just worked the plastic knife around each edge until it fit, then pushed in the locking strip.

I have a leaky windshield in my 70 Charger and am looking to redo it myself. The gasket and glass were new when the shop installed it. It has a lifetime warranty but I would rather learn it myself. I've installed 3 windshields in A-body cars and got them to seal right.