While yes, Granada (11" rotors) are a popular only conversion for five lug for the MII. To me those look like a Granada rotor. I second the GM calipers, probably Camaro. Again a popular "cheap" upgrade because the GM ones normally have a larger piston during that time was about the only upgrade for early Mustang brakes. As a side note a vast majority of "parts guys/gals" at any parts store are rarely going to have a clue. Their job it to type in your info on what you know and provide options, not figure out random parts. They do not have that kind of expertise. To qualify that remark I am, and have been a sales part manager for 15 years. Gearhead a much longer time.
How to Make a Granada Spindle and Brake Swap Actually Work
Like this: "Brake kit for 1974 -78 Mustang II and 1974 – 80 Pinto/Bobcat spindles. This kit adapts larger 11″ Granada rotors and 1982 – 92 Camaro (or equivalent) calipers for a simple, effective, and inexpensive brake upgrade. Some 14″ wheels will clear the calipers. 15″ wheels will clear. The 1975 – 80 Granada rotors move each wheel out 1/4″ from stock Mustang II rotors"
https://welderseries.ecwid.com/Musta...tors-p50694270