The top hole is not for the wheel cylinder, that is the anchor pin for the brake return springs. It's the 2 smaller bolts that actually hold the wheel cylinder on. The backing plate is held to the spindle by 4 bolts, should be 2 short and 2 long bolts, 1/2' diameter. The GM Duo-Servo brake system (Bendix style) is more or less the same from 1951 to 1999, with the addition of a self adjuster in the mid '60s.
