A Muscle car should have letters behind it's name, like; anything with a; G,A,R,S,T, or X, at least; like
GS,GSS,GT,GTX,R/T,R/S,S,SS,SX, AAR, etc.
GTX, and AAR, are great cuz those have 3 of the key letters.
Or it should have numbers like; 426/440/383/340,etc.
Or it should have a readily recognizable name/nickname like; Swinger, 'Cuda, G-Tex, Sport,SuperBee,
and not like;Tank, barge, leadsled, grocery-getter, or 'Doba
But I think it all comes down to being able to smoke G60s; If you can't fit G60s in the tubs, it ain't a muscle car.
If you can't smoke'em into second, don't show up.
More muscle will smoke 'em right thru second.
Most muscle will still be smoking'em in third.
This allows unconventionals in, like; 440 Newports, or a big engine Polara/Monaco. Typically we don't think of these as Muscle cars, But if you ever get a chance to ride in one; pay attention to the speedOmeter after 50/60 mph. Once she gets up, that speedO picks up speed faster than most any factory 340 car ever did.
I got the surprise of my life in 1971 or 2, when I finally agreed to race a 68ish NP. I had a low 14 second, as most of them were, 70Swinger 340/4-spd,. But I set the cut-off point at the top of third gear which was 90mph. Cuz in 4th, my engine sorta went to sleep, trying to pull 3.55s.
From the start, I got way out ahead, I mean waaaaay out. I was thinking this is gonna end in my favor. I yank the stick into third and check my rv mirror, and the spec of Newport is now nearly filling it up. At 85 he is pulling to my bumper and this thing is really moving. At 90 he is in my passenger side door. HONK!HONK! in the nick of time.
Now; my car Trapped at 98mph and change. In the quarter, he would have annihilated me. I learned a lesson that day; stay away from 440s.
So, is a 440 equipped 68 Newport a muscle car? Not in the usual sense. But I bet this one would have ET'd close to or in, the 13s. And that is very definitely in Muscle-Car Territory.
Funny thing is, I don't remember this NP having any letters, or numbers, in it's name, nor a nicname........ well, it did after that day. To me it was; "Bad Medicine"