Last year I went to the Mustang Owners museum in North Carolina. All of the mustangs there are either the museum owners personal stock, or lent by mustang owners around the US. Stock changes every 6 months or so and they even have one of the oldest (by VIN that weren’t showcase preproduction) mustangs as a “barn find”. 
The people that run it are obviously knowledgeable and were a blast to talk to. When I went they mentioned that they were trying to acquire the full scale clay S550 model. It’s possible that it would have been the one that was used in the documentary “A Faster Horse”.
For some engines, yes (see: 426 Hemi, 428 Cobra Jet).
However, in the early 1970s, horsepower ratings were changed from gross to net, so most older engines were actually overrated (by about 10%).
True. And the swap from leaded to unleaded didn’t help. I made a poor generalization and definitely meant the hipo motors meant for homologation.
If I can find that article, (which at this point is admittedly old) the dyno tests that were performed put out higher numbers than both standards.
But I can’t remember the dyno meter, and at this point it was probably a mustang dyno with higher numbers. Making the results meh at best.