It's really not how fast the cars were, it's how they went fast.
Muscle cars were generally drunk with torque. My uncle's 66 SS396 Chevelle left the biggest impression on me. The cars were quick, but you can't get the same pinned to the seat feel with today's cars as you did by merely depressing the pedal. Even an LSI car just ain't the same.
Not only were they torquey, they had those special sounds that new cars don't produce. Like that special wine of muncie coupled with the howl of a 12 bolt posi and the moan of big block breathing through a Holley. These cars were special, and they were built for young people in a special time in America.
The notchy feel of a big chrome four speed, and the smell of vinyl seats, sunoco 260 and maybe the tinny sound of an AM radio.
Now those were REAL CARS!