Though it has not happened to me (yet) my mechanic pal Gareth has come across on a couple of occasions water pumps where the impeller has separated from the shaft and is not therefore actually circulating the water.
None of these failures were on a Bristol or Chrysler engine but it might be worth checking.
Also as you have the 400 block with 383 heads are the Head Gaskets the same, could a waterway be blocked by the gasket?
It does seem a very short distance to get that hot, I recently had a Cadillac doing that when the water circulating wasn't actually that hot, the boiling effect especially in the expansion tank with water being forced out of the overflow was due to the system over pressurising, I my case the culprit was the head gasket failing but other owners warned about porosity in the block which should not affect the Chrysler engine but also cracks in the block and head which could and can give similar effects.
I also remember a freshly rebuilt 411 at the event at Filton a few years ago which was doing the same thing, the owner was going to have it looked at by Bristol cars so it might be worth asking Brian how they cured it.
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