When you install power brakes, the clevis connects to a different hole on the pedal (lower hole for PB, upper hole for MB), letting the pedal sit closer to the floor. In addition, there are two holes for the brake light switch bracket to mount into, so it has to move to the other hole so that it is aligned with the lower pedal position when you switch to PB. The 65 Chassis service manual shows all this in detail and which holes to use for PB. Also note that there is a return spring on the pedal with manual brakes only; the return spring is omitted when installing PB.