It has been decades since I've looked at this closely but my recollection is some lower body parts from 72-73 interchange onto 69-71 with only minor mods. The one thing mentioned in the video which I learned doing my Polara back in the early 2000s was do as much of the panel replacement/repair as you can on the car rather than on the bench. If you have time and patience fabricating your own replacement panels out of sheet metal is also the way to go. I therefore had no need to hunt for unobtainum parts.