I have done 2 windshields and a few rear glass installs in my life.
I just did a 1974 swinger and I used the old original gasket. I used zero additives or sealants of any kind and I have a perfect seal around my glass. A few pointers I have picked up.
1) Make sure your inner lip is completely clean, straight and has good paint all around before you start.
2) Use Plastic sticks ONLY as your pry tools. I got mine free from the place I bought my glass from
3) Lots and lots and lots of warm soapy water makes the job easier, and messier
4) Put the rubber on the car and then work the glass in to one side then bit by bit fold the rubber over the glass. 1/32" of progress at a time is huge! It gets easier as you go
5) once the glass is in, and still slippery and loose in the channel. SLAP THE HECK OUT OF IT.
6) Open handed palm smacks quite firmly on the glass, all over it to get it vibrating and take the tension off it.
7) If you do not slap it in, you get a perfect looking install that is likely to crack from stress before you circle the block the first time.
