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”.
Forgot to mention, this one is a 1970. So it’s got a little more fat to it.
Fun fact, pre-malaise era cars were massively underrated on power. Some manufacturers (on these semi race spec cars) quoted half the power they actually made just so they’d be able to sell for stock car homologation.
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.
I read an article in like 2011 where a group of engine builders rebuilt to factory spec and dyno tested the big 3's performance motors, each one of them produced over 700hp, not the ~350 they claimed haha. I wish I could find the article now, it was a great read.
I reckon they could only run with that claim as long as the tires of the time were still crappy! Now imagine any of the Big 3's big hitters on some brand new Pilot Cup tires - my god.
Underrating cars is still very much a thing.
In the 90’s is was the Japanese “Gentleman’s Agreement” that no car made over 276PS. The Supra TT and GT-R both made 276 on paper but would routinely put down 350+.
VW does it now. My MK7 GTI is rated at 220hp, 258tq at the crank, but it puts 230hp/270tq to the wheels.
It’s mostly an insurance thing.
I remember that article! I’m fairly certain it was Engine Labs.
The biggest way they cheated on these cars was rating them to stupid low rpm’s. Like Chrysler rated the Road Runner “peak power” at like 4300rpms.