It just depends on who you talk to. Muscle cars mean different things to different people. You can even go back further than the Chrysler 300 and find what some people consider a muscle car, even back into the 1930's.
Generally speaking though,,,,,the term muscle car didn't really come around until the 60's, and the 64 GTO gets most of the credit for starting that phrase. Doesn't necessarily mean it was the first, but a good example of one. Larry.