This is a common problem. The fix is a simple as cleaning, grinding, spraying rust prevention/treatment chemicals, then filling with water proof filler, to all and all out replacement. If replacing, first look for a good donor car that can be hacked up and welding into yours. If not available, most vintage type vendors like Paragon can sell you most of the windshield frame replacement sections. You can no longer get vin rivets, so don't damage your vin tag or rivets. It at all possible go in behind and grind the rivet out from the back and remove the vin tag for your rust repair. When you are done you can epoxy it back together with the old vin rivet head.
To gain access to this area you will have to pull the glass and all the glass frame trim inside and out. Pull the dash. Pull the windshield wiper arms and transmission. You may have to pull the doors. You may have to cut down into the top rear corner of each front fender to expose the windshield cowl area. Now you can sandblast, weld, paint, epoxy, etc, then glue it all back together. Obviously this will result in having to repaint at least the front clip since you whacked into the upper corners of the fenders and peeled them back.
They don't actually sell those rivets anymore. They list them in both the e-catalog and paper back, but they can not be purchased. It was brought to a halt last year. I was lucky enough to get some right before the federal government shut them down.