My 76 had a bad sheet metal crimp from the factory in the upper right corner. (This happened while Keith was on vacation.)
This meant the the slightly upturned edge of the opening caused the seal not to a "seal" properly. If you have someone take the windshield and seal out you should check that area. In my case, I took the truck back multiple times to the installer before I went out into the shop and saw the problem. They had been using more and more goop on the seal and that was never going to work. I had them hammer down the offending area and it never leaked after that.