With the windshield out the welded pinch seam needs some prep work. It should be fixed if rusted out. then a few coats of sandable primer and sanded down so its smooth. Then the 3M paint on Butal rubber sealant painted on and dried, before the new gasket is slid on. I then slowly lift the gaskets edging and smeer some of the sealer under.

Now getting the glass back into the channel you will be able to use some of the language skills you leaned as a teenage kid. I used Rylube rubber lubricant in the past. A dull putty knife with the corners rounded over helps lift the gasket around the glass. Lots of fiddly work going back and forth to seat it all around.