Door Rain Gutter on rear of 69 Dart - Rust issues

I also don't believe you need to be overly invasive with the repair. I'd clean it out as best you can, spray some rust converter, fill and move on. I used what appears to be a very similar product in applications like yours with great results that has held up over time. It was JB Welds Steelstik epoxy putty.

SteelStik Epoxy Putty Stick