1970 Duster front disc?

Yup, those are 73+ big bolt pattern brakes.

There is a difference in the calipers, the '73-75 A-body calipers had a piston bore of 2.6", the other models (B/E, FMJ, etc) and the '76 A-bodies had a piston bore of 2.75". They use the same caliper brackets and brake pads though, the calipers even look the same externally. You have to put them side by side to see the difference in the caliper bodies.

Also, you could get a '70 Duster with factory disk brakes. They would have been the small bolt pattern, 4 piston caliper Kelsey Hayes set up.