Please help me identify a cure! (Bodywork Newbie here)

I'd need a close up look at the cowl, and I'd want to inspect the backside of all of it, especially the areas around the vent boxes and drains. From the picture I have a hard time seeing if that's just deep pitting, or if it actually has pin holes. If there's pin holes it has to go. If it's deeply pitted but not pin holed yet, well, I'll say it should go and leave it at that. Pulling the whole cowl apart is a big job. As for the adhesives, well, I wouldn't use panel adhesives on a car that was designed to be welded, just like I wouldn't try welding on a car meant to be put together with adhesives. Unless you're going to run an FEA on one of these chassis to find out if a panel adhesive is really suitable for the joints you want to use it on, and which one is up to the task.

The trunk gutter is shot, and definitely needs to be replaced. You'll probably have to replace some of the metal on the quarter side of that joint too by the looks of things.