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I would venture to say it would be almost impossible to overheat an NP435 unless you were towing and hauling above your max GVWR and GCVWR and were in hills where you had to run in lower gears.
Keep in mind the NP435 was used in trucks as large as the F600, and in that application is almost the same as the pickup models other than a larger input shaft afaik.
As long as you are within normal loads, and as long as you have no bearing issues, the temperature of your NP435 should be way down the list of concerns, and perhaps not a concern at all.
Having said that, you must not tow a vehicle equipped with an NP435 without pulling the driveshaft like you would with a auto trans. More on that here: