Apologies for my obvious confusion, but how do you know this is a J65 rear end? I agree that the 3.31:1 ratio would suggest not originally from a Z, but maybe a road race application? Especially considering low level of 'general' public awareness of RPO Z28 in 67.
The PR application stamp only confirms metallic brake linings and final ratio, not what type of front brakes the car it was assembled in had.