Thanks to Rick Palmer for sharing this info he done on his Falcon
First things to do are to remove the hood, fenders and windshield, and wipers. Then with a good spot weld cutter, (I used one from Eastwood), and several extra cutter ends, cut all the spot welds. I recommend this cutter over any regular drill bit. It will center itself and stay on center and will help not to cut too deep and thru the inner surfaces. Then a wide, flat chisel will be needed to separate the cowl from the car. once the cowl is removed the rust will be visible. Use caution around the wiper hole. This should generally be in good shape but if a repair has to be made there make sure the measurements are on. I used a plastic mallet, also from Eastwood, to shape the new pieces while welding them in. This mallet really worked great for this. The usual grinding, wire brush and shaping will also be a big part of the job so I hope you have a good grinder. After all pieces were welded in then I measured and cut the hole for the new vent can and welded in place. Then I grind the area and leaded the repairs. I prefer lead to plastic filler for a more permanent repair. One thing is that it will all be covered and out of sight so the repair doesn't have to be a perfectly smooth one. As long as all leaks are fixed it won't matter what it looks like. I covered the surface liberally with Coroless also from Eastwood, or any good rust prevention paint or coating. Once all was dried I put an auto body sealer around the wiper holes and set the cowl in place and welded the cowl back in. I use a wire feed welder for sheet metal work. Whatever works for you will be best. once the cowl is welded in place then grind all welds as smooth as the originals were. (They were visible too) mainly get out the humps and make sure all were solid welds. then you can prepare for the painting and reassembly whenever you are ready. No more leaks on the floor board and rug.