For a number of years Pantera's have been unique where tasetful driver modifications have not adversely affected price, however even with the 71-74's there is a contingency that I would say has grown that is pushing for things to be stock. I think part of this is because there has never been a large club or judging association such as SAAC for Shelbys or Bloomington Gold for Vettes and the host of other marque judging associations. Panteras have avoided that limelight and scrutiny.
This Shelby Pantera you're referring to is unique in a few respects. One obviously being Shelby ownership, (and the seller is asking plenty of money for that part). Two being that it's a post Ford production car and was made in much much smaller numbers and, three that it is a narrow body 83 without the GT5 package. Listed as being the last narrow body made, which is also significant. This car is very stock and even has the Amerisport made air cleaner box.
Kirk Evan's is on here time to time. Being that he is Amerisport industries I would think he could answer some important questions for you.
To answer your original question on this car, I would say any changes would very much so affect value. In my opinion they are asking a large premium as is. If you are looking to do a color change and an interior change you are better off going to find a 71-74 for far, FAR less money (that is not historically significant) and make it what you want. I see no value spending that money (or even $100,000 less) on that particular car to then change it anyways.