You can have the windshield gasket "wet sealed" after it is installed. I have had this done to my truck and suggested it to another friend who had leak problems. Makes a big difference.

New rubber will not stop leaks necessarily so be prepared for this.

Wet sealing basically is burying the gasket/body/glass in a sealant very cleanly so that it is not noticeable looking at it. It solved my problems.

http://www.oldgmctrucks.com