To me, and by definition, a muscle car is a 2 door small to mid-size car with a powerful V8 engine. The '64 GTO was named the first muscle car when Pontiac put one of their most powerful engines into a mid-sized Tempest. Some say that you could classify some of the earlier cars as muscle cars because they had similar configurations (big engine in mid-sized car), but the GTO is still arguably the first muscle car. Over time, the muscle car has changed its ways with technology (mostly more efficient and fuel injected), and I call them modern muscle cars, like the new Challengers, Camaros, etc... Also the new Chargers, 300s, Cadillacs, etc... are classified as modern muscle in my book even though they don't have 2 doors, but that is also something that changed over time. The 5.0L Mustang in the late 80's is what brought on what we see today as far as modern muscle. I would classify the 5.0L Mustang as the first modern muscle car, and it should be in any "Real driving simulator". A pony car can still be classified as a muscle car, because of the definition of a muscle car, so to me, the early Mustangs, Camaros, and Challengers are still muscle cars because they had the option of a powerful V8 in a small to mid-sized car.
*Edit* The Shelby/AC Cobra is more classified as a widow maker. I mean come on, a small light weight British aluminum bodied car with the most powerful engine that Ford had to offer at the time. I would call it the first Hybrid Widow Maker LOL.