If you are mounting a cooler to cool your trans temp, WHY would you route you lines through a radiator to heat the trans fluid up? (195° thermostat?) Where I am, I certainly don't need a trans warmer!
Use a quality plate style, as a stand alone.
(Mine is mounted inside the fan shroud, so the engine fan pulls air through it.)
My WAG is that the coolant exiting the radiator is far cooler than the water entering (190) and water is a very efficient heat sink. That gives the trany fluid a head start before entering the air to fluid cooler .