Careful (but ad hoc) analysis of the head and cooling system revealed a serious design flaw in the head cooling system, amongst other things. In short, when the engine is cold, there is no (zero) coolant circulation in the head, and the thermostat, rather than being thermally intimate with any combustion chamber(s), is in a thermally-isolated pod in the front of the head. Head temperature skyrockets, likely with scattered hotspots, until convection and/or conduction transmit enough heat signal to the thermostat that it opens. Once open, temperature regulates normally. This situation is easily observed on a cold engine: start and idle. Feel the cylinder head with you hand; at some poin it becomes too hot to touch. Feel the thermostad pod: it remains cold. The fix is also below.

The water pump wants to suck water from the bottom of the radiator, push it up the block, and into the head; the head is filled with many passageways around all the combustion chambers and exits out the thermostat, into the top of the radiator.

There are two likely fixes, one easy one harder but "correct".

Easy: Drilling a tiny hole in the thermostat is common, it lets air bleed up to the radiator tank. A "big" hole (1/4"?) will let a decent amount of water flow. I intend to have that water flow past the actuator button on the bottom of the thermostat. Therefore, as the head warms up, head water will flow over the thermostat, "telling" the thermostat how hot the head is. Water circulating will mean even temperature distribution.

The downside is it will take longer to warm up and if the hole is too large, the head could run below rated thermostat temperature. But it's just a thermostat, easy to experiment with.

Correct: Re-plumb the head such that the water pump circulates water through the engine, not the radiator, as it warms up, and have warming water pass over the thermostat. It's not that hard, and with some effort, you could probably do it with the head attached to the engine in the car.

Drill the floor of the thermostat pod for a 3/8" NPT fitting, run a hose from there to a "tee" placed in the water pump's heater suction line. The water pump conveniently has two suction inlets; the large one from the bottom tank on the radiator, and a smaller one (5/8" or 3/4") from the heater. It is a trivial matter to tap the smaller line for a tee. The water pump will now draw water up through the block, into the head, through the pod, past the copper button on the bottom of the thermostat. Problem solved. Well I hope it is. News at 11.

Having the heater on will lower flow through the head, but it's hard to see how it won't still be a vast improvement.