[jhv48]

I'm having trouble wrapping my mind around this one.

So, engine rebuilt, total .60 overbore. Engine overheats but water is flowing correctly, thermostat is functioning fan is turning on at appropriate temp, radiator is shrouded. Now, the right bank plugs are white (doesn't necessarily mean a lean condition. Could be getting steam cleaned by water leaking into the cylinders), while the left bank show rich conditions. Dual plane intake should equalize, somewhat, the fuel distribution to both sides of the engine, thus ruling out the carburetor and you have eliminated a vacuum leak. Timing would effect both banks as well. Since the temps appear to be higher on the right side, I would borrow a laser temp thermometer and see how the right side header temps compare to the left side before I did anything else. My guess is that they're about equal. If the right side is significantly higher, then I agree, the head is coming off. Something is causing that side to reach higher operating temps. Either a water jacket is blocked by debris or a gasket moved during installation.

If you were losing any water, then I would suspect a head gasket leak, as the plugs could be getting steam cleaned by the water in the cylinders. That's why they still appear brand new.

My bet: intake or head gasket leaking on the passenger side. My first choice would be head gasket.