Even radiant heat loss from an external cooler is better than nothing. And 86tuning is right; put the cooler on the hot line heading out from the trans. The cooler will shed most of the heat, and the rad can soak the rest or warm the fluid if need be (cold-weather wheelin, the engine coolant will bring the trans temp up!).

Obviously, make sure you secure all your lines and double check that nothing is rubbing on a body panel/chassis part that could rub through.