The 11.75’s are also a factory Mopar brake, just for the later B/R bodies.
So the DoctorDiff Stage I brakes are the standard 73+ Mopar disks and are 10.95”, the Stage II brakes are the 11.75” Mopar disks. Stage III is a 13” rotor with PBR mustang calipers. Stage IV is a different set of 13” rotors with Viper calipers. The stage III and stage IV kits require 17 wheels, and there are probably some 17” wheels that will not clear the stage IV kit.