Overheating

[Claude]

Can someone provide a checklist of what to look for when a V8 bristol 411 overheats? It has the original 383 heads on 400 block that was added sometime in the past while it was still in the UK.

So far:
- Rebuilt the engine, carb, etc
- Rebuilt the radiator
- Tested the thermostat - opens at 180F
- Water pump is not leaking
- Radiator coolant looks good and does not leak

Drive less than 1/2 km up the hill from the garage, engine is boiling. Engine has not been driven since rebuild. Car has been sitting in garage for over a decade. Starts instantly, idles smooth.

Any thoughts?