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I have in a somewhat sad 37 120. I'll save the story of how poorly the brakes were serviced for another day. This rancid thing would start making steam after traveling about 1 1/2 times around the block. There were muddy deposits in the neck of the radiator and the air blowing back off the radiator while on fast idle was not all that hot.

Well,I thought, this thing must be horribly plugged up. But just to be sure I will check the thermostat. Oh, the thermostat is MIA; nothing but a big hole. So I installed a nice modern 180 degree stat as that was what I had on hand. I slapped it back together and poured in my oxalic acid to begin the slow process of dissolving the gook. This should take hours.

The car never got hot again! It came up to temperature, the thermostat opened, and intense heat rolled back off the radiator as it should. This was long before the acid could have done anything.

Simply put, the car needed the restriction in the cooling system to make it cool properly. I'd heard of this many times, usually in regard to flathead V8 Fords, but never seen it so dramatically. I am still running the acid in it to be thorough, but immediately after installing the thermostat the car became capable of climbing a hill on a 95 degree day. That was 100% out of the question before.

So, a word to the wise as you chase heating problems. Don't ditch your thermostat. That will make it worse.

Since this is cooling.

The 400 I don't have heater in. I run hose from block to water pump.

Engine runs cool. But its will start to climb stuck in traffic.

Could running hose to water pump. Cause more heat.

Hooked up as you describe, the hot water is shooting out the back of the right cylinder head and is sucked right back into the water pump without ever coming near the radiator. When everything is moving that may be OK, but when sitting you will be recirculating way too much.

Just pinch the hose with a Visegrip for now.

That is a common problem with the 6th series cars; the water goes too fast through the radiator to have any effect.

It's also a routine ASE test question.

On the cars with shutters Packard seemed to address the issue by using relatively small diameter hoses on the cylinder head, but given the thermal inefficiency of the V-12 it would be interesting to test thermostats given that motor was designed before the modern pellet type.

I suspect it may have had an air pocket in it, if there was "muddy" stuff in it. Draining it to change the stat and refilling, may have cleared what ailed it. Common issue when things aren't full and once you get steam, it keeps making steam. The wall temperature where the pocket is, gets too hot and creates steam pressure keeping the liquid away.
I'd expect without a stat, it might not run good when cool as it won't get up to temp for the oil, combustion, etc.
That stat is not controlling the temp to 180, it's not fully open til it gets to 180, to allow the engine block to heat up. I don't think there's a whole lot of differential pressure once it's fully open to make a difference for restricting flow. They aren't that tight closed either. Some have weep holes to let air out. But would make sense if there was an air pocket, nothing to restrict the steam going to the radiator.
It's a centrifugal pump, so will only flow what it can flow with the available pressure drop at the rpm it's turning.
But agree they should have one. Hope it resolves fully.
Flush and put coolant with rust additive after it's "cooked" out. Glycol doesn't like acid. It's breaks down faster.

Yes, the cooling system requires restrictions in flow. I remember hearing that some folks place large washers into the cooling systems of Ford flatheads. These washers restrict the flow and the engines run cooler as a result. This point was proven to me by an operator at a heating/cooling plant. The centrifugal chiller at the plant transferred its heat to the cooling towers. The valve to the cooling towers was never left wide open. The operator showed me that if he opened the valve too wide, the chiller would actually LOSE efficiency. A couple of minutes after opening the valve slightly wider, the chilled water temperature would inch up. Obviously, the valve setting was predicated on the outside temperature and how much demand there was for chilled water. But, by slowing down the flow through the unit, more heat was able to be transferred. If the water ran through the cooling tubes too quickly, less heat was transferred.