I'm not an expert either, but I agree the timing should be higher. The 1968 Pontiac service manual says 9 degrees for a V8 , 6 for a six cylinder. I'd start with 12.
You may have the idle screw set so high, to keep the engine idling with the low advance, that the throttle plates are out of the idle circuit and into the transition circuit. That may be why you have no change with the idle mixture screws.
Been wrong before.