I see strong evidence that this car is a rebody, but I don't see clear evidence that he did it or knew about it.
It's entirely possible he knew he wanted a 70 Challenger, started collecting parts (none of us have ever bought parts for a car we didn't own, right?
), and then bought the car somewhere, either as a pile of parts or as a project that had already been rebodied.
His aggravated response could be expected if anonymous people began telling him that he and/or his car was a fraud.