I would say sure a muscle car to me is more the motor and size (footprint) of the car. Many 66-70 Chevelles SS did not come with a Posi, but a 396 was standard. I would classify all them as muscle cars. I think where I take issue, is pony car vs. muscle car. To me the Z/28 and all small block camaros is more pony car than muscle car, and for sure the Mustangs would be more pony car, unless they had a big block.
Also IMO while I would say the 61-62 impala/bel-air with the 409 is a muscle car, since there was no other early chevy alternative, I tend not to put 65-69 Impala with the 396 in the muscle car category, because how big and heavy they are, and you had the chevelle, camaro, and then the BB corvette as a go to.
I am sure someone could make the argument for the dressed down 2 door post cars (bel air) as a muscle car with a BB, I would say that may fall in the category as well.
But is the 1970 454 monte carlo, still a muscle car? I would say no, more luxury car with a BB.
Thats my 2 cents, and may vary a little when discussing different fords and mopars.