PBR calipers came on the 99-up Mustangs. To mount them on earlier spindles requires a slight modification of the spindle. 96-up spindles push the wheels out about 1/4" per side. The 94-95 spindles are the preferred part. If you look on the backside of the upright, they should have part numbers that begin with either "F4" or "F5" on them. If they have anything different, they're later ones.