Hi Tony
Interesting thoughts on cooling issues, takes a bit of careful thinking to nail down the causes and issues. Agreed with what you are saying but still trying to figure out what is going on with my Pat 12.
Concerning flow testing, when I did the water pump conversion on the 261 engine before putting it my C60S actually did flow testing both with the stock 261 water pump and with adapter plate and the 216 water pump.
Set up the engine test stand with a large tank at level of the standard radiator so the water came out the top outlet and dumped into the tank with 22 inches of standing water in the tank same as the radiator. Of course with the thermostat installed the flow rate is control by temp and RPM. Results with the two water pumps came out almost identically so it probably the thermostat throat that controls the flow rate. The at 500RPM flow is 2-10 gallons per minute at 1000RPM flow rate is 8-20 gallons per minutes.
After two hour long test cycles to day with the 235 I'm getting strange results. I have two water temperature sensors one in the normal location at the rear left of the block and the other into the water pump housing. At 500RPM the engine seems to stablize with the rear reading 170F and the return reading 150F good 20 degree delta for the radiator. But when you bring the engine up to 1500RPM rear is reading 170 and the return is reading 170. To further confuse things though at 1500PRM the water going into the at 200-210F (read with IR gun) and returning at the bottom of the radiator 198-175
The other thing that was going on during these test was the air temperature in the shop went from 74-80F.
I knew there was a reason normally do engine testing and break in running in the winter heats the shop up nicely.
Will have to figure out how to display the data so you can take a look.
Cheers Phil