I owned a 1975 Duster 225 Six loaded paid $3600 new had a factory wind type sunroof. It was white with a white interior the only one I have ever seen. However at no given time was it ever ever a "musclecar" with a slant six. It was continually in the shop with electronic issues. Anyway, the body type does not make a musclecar and even if the year I had had a V8 it was not a musclecar. None of the earlier slant sixes were a musclecar either... regardless of body type unless you ordered the 340 V8. An earlier 340 V8 Duster, Demon was a musclecar. 383.426,440's were musclecar engines, but a 440 Chrysler 4 door sedan like my dads Police Intercepter was still not a musclecar. I have owned a 1950 Plymouth Business Coupe and a 1964 Plymouth Savoy with the pushbutton automatic. As they were both sixes they too were not musclecars. If a 340 or larger engine was transplanted in one of them they could have been. But again body type means nothing. Chrysler made great engines and great looking cars. The most reliable Chrysler I ever had was the 1964 by the way.
Your Cadillac is not a musclecar or a sports car. It is a luxury car. Since you drive a Cadillac I wonder why you are hanging out on a Mopar review. Having owned quite a few Mopars I felt my input might have value to others. As I stated earlier my dads 440 unmarked detective Intercepter 4 door sedan also was not a musclecar. However in all fairness during the 50's you could pull a Cadillac engine transplant it and make a pretty cool "Hot Rod" This was not uncommon. The Northstar in the newer Cadillacs is pretty respectable by the way! But again they are luxury cars not muscle.
I love Mopars, but I also have to vote NO on this being a muscle car. My '73 318 Charger is not a muscle car; my '71 340 Barracuda might be marginally considered a muscle car; my '67 383 Chrysler Newport 4-door was not a muscle car; my '64 Slant-6 Dodge 440 was not a muscle car. Most accepted definitions of a muscle car involve all of the following: 1) stock V-8 engine of either large displacement or high RPM that offers more than 350 horsepower; 2) 2-door coupe, sedan or convertible; 3) rear wheel drive; 4) late 60's to earliest 70's vintage; 5) possibly stripped down or utilitarian accoutrements for the sole purpose of offering performance at lower price (i.e. Roadrunner with rubber floor mats and bench seat). If your Duster had come with a 340, it might be considered a muscle car, although a '73 340 was not the fire-breather that the '71 340 was. Even so, a Slant-6 Duster is a neat little car, so enjoy it for what it is.