When I remove windshields from b-bodies, it always looks like there's sealant all of the way across the lower channel of the windshield. I had a local glass shop install the windshield in the 67 hemi GTX and it leaks like a sieve. Also, the inside of the gasket is puckered badly where it lays against the dash panel on the inside. We ordered a better gasket from Chuck's which is supposed to be the premium quality gasket. I was planning to add sealant all of the way across and part of the way up the sides.
As for the sealant for the front...are you talking about the 3M flowable in a tube or the rope style butyl? Looks like 3M is only making one non urethane sealant for windshields now.
I have three winshields to install. The GTX, my Daytona clone and my 68 runner that I just traded for.