oh i hear you on the rust i have alot of rust/rot what i was planning was drilling spot weld hols along the flange then prime and paint the mating surfaces then setting the panel on and tracing the spot weld hols then cleaning the paint on the trace marks then welding the holes a bit extreme but the paint will help the sealing bare metal i did this when i remove my wiper tray to clean and paint up under it and it welded fine .