Ditto what c1800 wrote. Another attempt at pollution control is the 2nd throttle plate located in the intake manifold which opens when the throttle pedal is pushed far enough. The 2nd throttle directs the fuel/air mixture into a central chamber rather than directly to the cylinders each carb serves. The mixture at low part throttle then finds its way back to the intake runner leading to the cylinders. I remove those 2nd throttles and fill the remaining hole with a 5/16 bolt. The inside end of that throttle plate shaft ends in a blind hole. I grind the head thinner and use a sealing washer with the works installed from the inside. On the outside I double nut the bolt.
Keep in mind that if original the compression ratio of a '68 is a healthy 10:1 and requires premium fuel. If the head has to come off for any reason having hardened seats installed on the exhaust side would be a good move. - Dave