Couple issues on my new to me 65 dart…

Fixing the cowl rust is a big big big job. Not for the timid. That small rust spot is most likely much worse on the inside of the cowl. Its just what happens to these cars.

I would highly recommend keeping it out of the rain as much as you can . Dont allow any more water into the cowl .

You could pull that fresh air box from the inside of the cabin and just cover up the rust hole with fiberglass or whatever magical compound available , but its just gonna keep rusting inside the cowl until the metal is all gone.

Por -15 dries hard as a rock and is extremely difficult to remove. So if you put a bunch in there and then decide later that you want a re-do it will be a bunch more work for you to strip it and start over.