I am no expert body man, but would like to share this with you. I did a similar repair on a 70 Mach I. I really took my time and did the best I knew how with a body man of 20 years advising me all the way. In the backgound he was always saying no matter how well you do this its only a temp fix, and since your spending all this money on paint, filler, primer and paper just spend the extra $ on the right part. 2 years after repair, it cracked...more and more each month. Every time the car was driven hard it cracked more. In the end I ended up buying the rear wheel well sheet metal and welding it in place. Fix is now 12 years old and in perfect condition. JMO. Good luck to you in whatever you choose to do.