If you want to get fancy, you could put in a heat exchanger out of a diesel. They really pull a lot of heat out of the transmission. One of the guys on the DTRF pulled his out, thinking the transmission would run cooler. It was the only mod he made between two road trips pulling a camper, and he couldn't hardly stay in D without overheating it. And he forgot about towing in OD.
Overheating the motor might put more heat into the transmission, but for the other 99.8% of the time, it will cool it.