Dutch, give us some more info.
Is the car an automatic or a stick?
If the car is an automatic, does the radiator that you installed have a transmission cooler built in, and are you using it? Or, are you only using an external tranny cooler?
If the car is an automatic, what stall is the torque converter? Wondering if the converter is slipping excessively at cruise. Cruise rpm should preferably above the stall of the converter or very close to it to prevent excessive heat generation.
I would like to assume that the radiator is well sealed to the core support so air flow must go thru the core and cannot go around it?
When you remove the radiator cap and look at the tubes in the radiator, are they at least 1" wide?
Have you removed the thermostat and dropped it into a pan of hot water to check what at temperature it is fully open? (I have seen quite a few imported thermostats that don't open fully until a very high temp in recent years}
If you crank the car when cold and let it sit and idle, does it gradually get hotter and hotter? Or, does this only occur after the car has been driven?
Is the fan hard to turn by hand when the engine is turned off after the car has warmed up?
Now, if you are using the mopar water pump housing, I kinda doubt the pump is the problem as long as the impeller is firmly affixed to the shaft and not slipping. I agree the Milodon is a superior design but I don't think it is the culprit if working properly. It does not sound like the car is being run hard enuf to need a superior water pump at this point.
All cars run high temps under the hood. Cars that run headers run even higher temps under the hood. However, when the car is moving, air should exit under the car due to the pressurized air flow coming thru the grill and when sitting a properly sized/working fan will push it out. The factory engineers were usually knowledgeable and experienced enuf to understand air flow over a period of years so that mistakes were eliminated if made with successive models.
As you are running 4.10 gears, you are putting more heat into the system from the increased rpm but you are not the only guy running more rpm. It would seem that you are not pushing the engine so that it is mainly cruising so the load is not great on the engine and therefore it is not making a lot of heat as it would if you were racing it.
It runs hotter than would be expected on the road at relatively low cruising speeds which would seem to indicate a lack of air flow. In the old days, they used to tell us that a car did not need a fan once speed was up to about 45 mph as the air stream provided more than the fan at that point. I cannot say that is an universal number due to the variation of parameters involved. I don't know if you have any stretches of road where you can run a consistent 60 mph to see what the coolant temp does in that situation.?
Not knowing anything about the Porsche coolant, I sure would like to see what happens if you try 100% water in the system or as close to that as you can practically obtain without jumping thru hoops. Most modern cars don't turn the fan on until well over 200 degs F and I am wondering if there is a possibility where Porsche designed a matching coolant.
Sorry for too many questions but I have learned over the years you have to look at the system and not just the parts...
