The dark ages of performance
Off the top of my head I would have to say Camaros and Corvettes helped the cause.
I had a late model 2nd gen Z28 many, many years ago. Not exacty up to par with a '69 Charger with a big block, but it had a 350/4-barrel that was rated at slighty more horse power than the standard 350. It was really more looks than performance that set it apart from a 'standard' Crapmaro.
Grand National pops into my head as a muscle car of it's day...
I also owned a '86 Charger. I'm not sure I would have called that a muscle car, but was still a Charger!
Red