Heres my two cents:

I work at a local track and watch EVERY kind of car imaginable go down the track, I have watched many mustangs and gtos go down the track and sorry but they dont come close to no 13.10 or 13.50 or whatever.....PERIOD.... Im not quite sure of a cut off date but it was before the 80s and after the 70s for sure. You may think that turbo cars should be muscle cars or vipers are muscle cars or whatever, in my opinion they are a rich mans sports car. If its not sold at a swap meet then it aint muscle.