Duster 340 not a muscle car...

yeah subjective term for sure. Performance car? Musclecar? Or just a car from the musclecar era?

318/350/289 etc were standard motors

To me "musclecar" is the car with the most powerful motor available at the time for the body style.

So a 340 A-body would qualify. A 350 Chevelle would not be, as a 396 or 454 was available.

but whatever they're all cool....