While some cars have inner fenderwells that are just splash guards, on an A-body, the inner fenderwells are structural, an extension of the front subframe. It doesn't mean you can't cut them, you just have to be careful. I've seen a couple Darts that had fenderwell headers with too much metal removed and some pretty visible signs that the front end was sagging from a bad installation job. Things like what had to be minus ten degrees of camber and the like. Be careful with how much metal you trim out, and keep the corners round instead of square for best results.