The overall question is DO YOU NEED IT? Ideally trans temps of 210* and under are fine. Blindly guessing that your transmission needs an additional cooler is the wrong way to go about doing it. You can overcool a transmission, especially in the winter time. Most trans have to meet certain temp in order to shift into OD or lockup TCC.
I'd say if you're wanting to aid in prolonging the life of the trans, add an additional quart of lubrication by adding an external filter, that is unless you've verified your trans is getting hotter than it should.
I drove around a 95 Buick Roadmaster, with 3.73s and a 3400 9.5" diameter stall torque converter for 3yrs. With the factory intank cooler and factory external cooler, I never saw temperatures over 180* in any kind of driving application (well save for towing). Intank coolers can help regulate the temps when they get pretty hot. Granted I did have a 160* T-stat in the car, coolant temps were always in the 175-185 range.