74-75 (?) Imperials could have had factory rear disc brakes (on a 9 1/4 rear). Back in the 80's, I got a set from a salvage yard and put them on my 74 Road Runner (8 3/4 from a 69 B body).
In the 80's, I could get replacement pads and rotors. It wasn't a direct swap, but I don't recall all...