If it were mine I would be doing some major surgery to replace all the rusty panels. However, seam sealer will fill those holes. Undercoating would not be my first choice. You could also get some of eastwoods internal frame coating to seal up the rest of the inside of the cowl. I used it on my car and then ran a camera through the cowl and it had complete coverage. It took two cans to do the inside of the cowl.
