I do not want to insult your knowledge but based on what you have said, I understand that you are trying to get body parts.
As one who recently installed a "LIMA" (472") motor in a 1973 Grande, I can assure you that the body parts for the 1971 - 1973 Mustangs are readily available from numerous Mustang Parts houses, i.e. "Mustang Plus", "Mustang Unlimited", "Texas Mustang", "Virginia Mustang, "National Parts Depot" - - to name just a few. The only item that is apparently not avavailable "New" is the '73 front bumper which is different from the '71 and '72.
While you did not ask about motor fitment, it should be noted that since the 429 came in the 71, new replacement suspension suspension parts are also readily available.
Let's face it, the 1971 - 1073 Mustangs were MADE for the 429/460 motor. My next project is a 545 for my '73 MACH I which currently has a '70 428CJ. I built the Grande to get experience for the MACH I project. This was all done before I found this site. Based on what I have learned on this forum over the last couple of months, the 545 installation is going to be AWESOME.
The best of luck with your build.
Do the right thing!