Trying to identify my current spindles ‘73 Dart

You do have big ball joint spindles on the car, so your correct in that department.
But looking at the picture of the caliper, and where the rubber flex hose is mounted to the caliper, it appears to be a 73 and up, B body caliper.
So i think you have a 73 up B body disc brake conversion on the car.

And NO, your spindles ARE NOT considered to be 10 inch spindles.
That designation is for DRUM brakes, and not Disc brakes.