The Chevelle, GTO, 442, Charger-- those were all more like sedans or family coupes in body style. They were muscle cars that were based more on family cars.
The Mustang started the trend of the long hood and short back deck. It's more pronounced on the coupe than the fastback but the same basic lines are there. This is where the Camaro, Firebird, Barracuda, Challenger, Cougar, all played.
Corvettes were always... something else. I'd never consider them to be muscle cars. They were America's sportscar I suppose. Still are.