by def. A muscle car is one made from '64-'72 equiped with a powerful engine and not a run of the mill engine. Engine size is of little importance to relation to the term. So big block guys can sit it.
The era ended in '72 when compression ratios dropped like rocks, valve sizes shrunk and camshafts becam broom stick. Emissions became the hot topic and power killer. So, technically, my '73 cuda is not a muscle car be ause of the "detuned" engine even though it is otherwise equipped exactly the same as previous years.
Back to the engines a second here.
Just because you have a *** car with a mid 300 cid engine doesn't automatically make it a muscle car. A typical camaro for example came with a 327 or 350 in later years as a normally seen displacement size engine. There is absolutely nothing special about it. In fact, they basically littered the streets to the point of normal.
Now if you had one with a big block or the rare to be seen dz 302 engine, you got a muscle car. Same with the stang. 302 ='s ho-hum and so freaking what. Boss 429 inside? Hoooo yea!!!!
With that said, later year cars were updated with performance parts and given there muscle afterwards. Not born that way but transformed. This can be done to nearly any car. Though a neon will always be a tuner and a **** box by any real car man (or lady) they fall in line with the jap cars. "civic"
hot rodding them doesn't give them muscle car status and never will though the idea of pumping up the weak is for ever a hot roddlers way of doing what detroit did for us so many years ago and is starting to do again. While the hayday is gone and dead, never to return like it was, the hp wars are back on and the second coming of the muscle car is here and now.