Until this thread- I have never seen or heard of a transmission suffering from being too cold either, and exactly how were you made aware your transmission fluid WAS indeed not warm enough to evaporate moisture out of itsself? This would be more of a problem in Canada, than Arkansas, and then limited to the cooler months of the winter.
All irrelevance aside the subject here is not enough cooling and what is a figure to go by for enough?
You cannot accurately go by the GVW rating put forth on most of the coolers sold by nearly everyone who sells aftermarket coolers.
A rough generalization is still only a guess, but if you intend on running street with that and want to keep a better eye on it a- transmission temperature gauge with the sensor mounted in the transmission pan would be the BEST starting point. Several threads here on Hotrodders have indicated the pan is THE BEST source of transmission fluid temperature for accuracy. Only after you know how much over the optimum 175(f) degree temperature you are, can you make the most effective purchase of cooler for YOUR particular needs.