the a body 8 3/4 was only available in 5X4" small bolt circle from the factory, the 8 1/4 a body rear was only available in 5X4.5" big bolt circle, if your car came with big bolt single piston front disk brakes it would be supplied with the 8 1/4 rear ( 7 1/4 was available in both big and small bolt circle)
