Musclecar = "Factory" built Hot rods!!!
The 55, 56 and 57 Hemi 300's were the first Muscle cars, I remember seeing an ad for an early 300 and it had Muscle in the title and I think that is where the name came from.
The name musclecar didn't become well know til the 64 GTO came out and I think it was the insurance companies that started categorizing them as Musclecars.
That's pretty much the way it is, cars like my Fish, my Dart, your car and every other A-Body that started life as a slant 6, 273 or 318 car were sold as economy cars and that is all they are no matter what power plant we have in them. They may have muscle now but they are concidered Street machines, Pro-Street, Hot Rods or what ever you want to call them but to say they are a real Musclecar is a little far fetched. To me that somewhat degrades the real Musclecars. You can stick a Hemi in a Dodge Cricket but it doesn't make it a Musclecar, it's still a Cricket.