My 70 Dart

Well, I'm stuck. Part of me wants to cut out the rust and replace the steel. Another part says why not just remove the undercoating that is there, and treat the metal to eliminate the rust, then use a screen and bondo it. If I cut the panel, where do I get the replacement steel? Should it be the same gauge as the q panel? I can get a MIG welder, gasless, but I'm cleaning the heck out of everything anyway. I'm not taking the paint off the entire car, the rear valance is in real good shape as far as paint is concerned, and it's straight. I am going to rough it with 100g paper then paint it. Ok, thanks!