Rustoration Heeeelp!

You can bet that both sides are bad. Leaves and dirt get throught those slots and clog the drains on both sides of the cowl where the hinges are located. Your heater draws air from the cowl on the passenger side. There is a round deflector welded to the inner cowl to stop water from rushing into the heater box. You can bet that is damaged and will need attention. I would remove the fenders and check the drains. The seam runs below the windshield post on both sides. It is filled with lead that can be melted out with a map torch. There you will find spotwelds. You will need to remove the windshield too to access the spotwelds across the bottom windshield pinchweld. That lip just below your red markings has spotwelds across it. I just use a cutoff wheel and grind through the spotweld until it turns blue and the edges start folding up. That is when you know you are through the first layer of metal. Do this on all the spotwelds and that upper cowl will come off with little effort.

Open the vent under the dash on the passenger side and see if it is full of debris! You can reach through and feel around to find that hole you knocked through with your screwdriver.