I always understood Mustangs to be pony cars, not muscle cars. Also, I don't remember anything below 300 cu in being available in a muscle car. Some had 6 cyls available in a base model, but that kind of killed it being a muscle car. Ford pony cars had 260's and 289's available, but they weren't considered muscle cars.

Now, modern pony cars can leave old muscle cars sitting easily, but it's hard to beat the rumble of a big block muscle car.