3m windshield urethane is rated at 1200 psi and can elongate 750% before it tears.
it's strong enough to withstand the force of a air bag, they expand at about 100 mph, and not tear.
strength isn't the only requirement to hold in a windshields. is also has to have elasticity. if it doesn't have that you hit a big bump your windshield cracks. note windshield urethane can elongate 750% without damage.
also using silicone anything almost guarantees fish eyes, something you really don't want.
http://multimedia.3m.com/mws/mediaw...QEVs6EVs6EVs6E666666--&fn=TDS_08695_08564.pdf
ford, CARLITE the people who make the factory windshields, step by step windshield replacement recommends using ESESEX windshield urethane.
http://www.google.com/url?sa=t&rct=...0oCIAg&usg=AFQjCNGDVLNuufyQazvJKsnIWmUa2Ym0tA
back to the OP questions.
YES you can reuse the old gasket.
make sure you remove all of the old glass and urethane from the gasket. you could carefully cut off the bottom channel that slides on the inside of the glass, lightly tack it to the glass with some weatherstrip adheasive, then bed it in the windshield urethane, let the urethane ooze around both sides of the weatherstrip and you should be good to go.
most glass companies can do this without problems so it would probably be in your best interest to have a pro do the job.