We did one in a week end, Local guy Mike "dusterdawg" drove it in on a Friday and drove it out on Sunday night. Slant 6 72 Duster. He is a member here. My son charged him. $800 and we supplied the clip. we did not clean or paint it he sprayed it with a black rattle can before we put the motor back up in. We put the car on the lift when he came for bushings. It almost broke in half. He was diving it every day like that.
I always liked fixing them when they were bent bad . But when it comes to paying the bills I had to take the rust work that no one did.
The 2 pics are of a 4spd duster we did. He can do it cheap, or He does do it and clean everything removed including small parts. two car garage behind my house that my son still works out of. But I get to act like the supervisor.
So you can do it yourself with a little time and some tools. Members on here will guide you. But I would not repair that area you are talking about. I have been doing this a long time. Once that area rusts the rails are usually rusted bad from the inside out.
Clips are getting hard to find. That is why Dave "69383valiant" cut that clean more door up. Or he could just hate more doors? I tried to talk him out of it , He wouldn't listen. He was determined to give me the AC parts. I didn't want to make him feel bad. But he did have Jim standing on his shoulder holding a pitchfork saying "Cut it Dave Cut it"