Vibration dampener all in order? Possibly running extremely retarded if timing mark on dampener has shifted, causing ignition timing setting inaccurate.

Even with secondary carbs disabled, should get at least 5 grand on center carb alone.

What does your vacuum gauge read at wide open throttle at 3000 rpm. What does it read at idle?

What is voltage at positive coil wire post. Should be st 12 volts unless thru ballast resistor., then 8 plus volts.

No huge vacuum leaks such as power brake or pcv disconnected. Unlikely to cause this,,,never know.

Excessively weak valve springs. Forget if this has been checked.

I also like valve timing suggestions.