Coolant flow

It wasn’t cheap $$ that’s for sure. Flat plate shroud with high CFM fan. It didn’t take long for me to discover it had to go. You may have known this from the beginning of time, at the time I bought it I had to find an 18 inch fan with shroud that would fit my situation for dragracing and street use. For that it was fine, although I had to run it more than I care to. It was no good for Highway use in my application. And that style is what I am referring to which is the exact same thing you are referring to.
Here's another example where the shroud design was the killer.
1941 Plymouth. Not much room. @MopaR&D figured it out first.
Shazam! Took the shroud off, fabricated new mounts and took it for a spin. At 70 mph, 91 degrees ambient, it stayed on 180! I guess my homemade shroud was too shallow and too restrictive after all. Looks like it will stay cool when idling with the A.C. on, as well. Thanks for everyone help And suggestions!

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The devil is in the details. (Kind of like @92b wrote, what setup is a best match can vary with application, and the constraints.)

Chrysler Thermostats

1966 Dodge Dart Coronet FSM
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^^^^^^^^
Opening has a plus/minus allowance of 5 F degrees.
Fully open is approximately 20 F degrees higher.

1970 Plymouth FSM
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^^^^^^^^^^^^^^^^^
Opening temperatures have a plus 4 minus 3 F degree allowance.
Fully open is about 20 degrees above the rating.

Flow
Full flow to the radiator was not expected until the temperature gets up to 200 degrees.*
This creates a variable flow situation. It means in very cold conditions, the coolant will exhange heat through the heater core.
On the other hand, in hot conditions (or when the engine is under load for extended periods of time) the coolant will exchange heat through the radiator.
*With the later cars using 190 and 195 degree thermostats full flow doesn't take place until 210 and 215 F.

However, that's not all folks.
When engine conditions created enough vapor, the coolant system would pressurize.
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