I have not been able to slow this beast down. It idles at 1,200 rpm. Have new spark plugs, carburetors are in sync, timing is dead on @ 12 deg. BTDC. Idle adjusting screws are completely backed up. Any ideas???

 

You have a leak somewhere. IIRC there are no fewer than 17 vacuum taps from the 4 intake manifolds and they all go to various T's and connections that can leak. If you haven't replaced all the 40 year old vacuum tubing then you should do that first. How about your gulp valve, is it working properly? Your carbs also have throttle bypass valves that let air pass around the butterflies under high vacuum conditions. If those 40 year old diaphragms haven't been replaced then that would be the second place I would look.