Duster 340 not a muscle car...

Some old school fellas would refer to it as a pony car , as it is a small block. What would be considered muscle car back in the day had big block in a mid sized car. By definition, I would think the 340 Duster would qualify as it was a small block in a small car, or what was at the time considered a compact car. The 340 dart is definitely a small car for the late sixties/early seventies time frame. The Duster is slightly larger with the fastback rear. I know this may sound weird, but the OP and his buddy are both correct, they just have a difference in definition, if that makes sense.