best way to fix rust around rear window

Hello all:
I have a 72 scamp with alot of rust around the rear window at the bottom corners and the flat part of the fender by the corners. My question to all of you is what is the best way to repair? Find a donor car and use its corners, or shape some new steel? I'm kinda new at the entire removal and replacement of steel. If the doner car is the way to go how much do i cut out and if the new steel is the way do I have a professional do it. Now that ya kinda know what i'm looking at, any and all info would be appreciated.

thanks

kevin.