Classic = pedestrian version. Muscle = image cars of that time. IMHO, the beginning of the muscle car era began when OHV V8 engines started being more common, with multiple carburetion, fi, etc. I would have to say around 1957-ish.
Even though it is a land yacht, I definitely consider an early 60's 409 SS Impala a muscle car. I feel the same way about the early 60's mopars with max wedge engines. The list goes on...
Here is a link to the Pure Stock Muscle Car Drags:
http://www.purestockdrags.com/index....d=48&Itemid=53
Their definition is on that page.