Try a Derale. Comes with a 800cfm fan. Kinda of hard to cool a Trans if the truck is not moving. Temp while towing stays around 165. Temps while racing will jump to 200 then return to 165 after fan has been on for 10 mins.
These temps are with my built Trans. Keep in mind if your Trans is on it's way out your temps will be higher due to the slipping of clutches.