You might be able to find a few threads including pictures that show do to do that repair. Basically you have to make patch pieces of metal. A metal brake would be nice. A shrinker would be nice to make the curve in the metal. Cut out the old, weld in the new.
Here's some links
http://www.chevelles.com/forums/showthread.php?t=257225&highlight=windshield+rust+repair
http://www.webrodder.com/article.php?AID=296&SID=3&CID=7
http://www.webrodder.com/article.php?AID=295&SID=3&CID=7
Here's some links
http://www.chevelles.com/forums/showthread.php?t=257225&highlight=windshield+rust+repair
http://www.webrodder.com/article.php?AID=296&SID=3&CID=7
http://www.webrodder.com/article.php?AID=295&SID=3&CID=7