How would you fix this cowl rust?

There isn't really a stellar way to fix that without taking the cowl off, and fixing it right. She's had a good run, 48 years. Time for some rest-o work. Or block it off and hope for the best.

You probably have more issues you need to address at the same time, for instance resealing around the hats, and a undercoating job in there anyway so the other side doesn't look like that in 5 years.

Think about body panel adhesive for re-installation, it's what I'm going to do in my build, no more rust issues. Seals nicely. :)