I've got another idea. Whatever you do, the windshield needs to come out to perform this repair. That said, Eastwood Co has the best deal going on body panel adhesive. To my knowledge, theirs is the only one that can be applied with a standard caulking gun, which significantly cuts the cost of the process.
http://eastwood.resultspage.com/sear...29&submit.y=11
If you can fabricate some sort of patch as 69BBsuburban suggested, and be sure that all rust is removed before beginning, you should be able to create a permanent repair without welding.