Steve,
External coolers to me are concidered secondary or supplemental. I see allot of people just running the externals and small ones at that. They dont cool the fluid off as much as you think maybe 50 to 70 degrees at best, If your driving this thing on the street or have a high stall converter I strongly recomend a rad w/ a trans cooler in it, they really bring down the temps way more than the externals do. Think of this analogy, if you burn your hand do you wave it in the air or do you run to put it in some water? Even though your radiator ( when hot ) maybe 190 plus degrees the fluid coming from your tranny maybe in the 250 to 350 range especially if running a high stall converter or doing some city driving, if you dont think the fluid gets that hot grab one of the metal lines after stalling it up or driving it around awhile. By running it thru a water heat exchanger you cool it off 100 degrees or more, then you can run it thru a external cooler to cool it off about another 50 dgrees or so. Now you have about a 150 degree drop in temp, Heat kills trannys and converters the cooler you can keep the fluid the longer they will live. If you do only run a external get the biggest one you can fit in FRONT up the radiator
( old A/C condensers make good external coolers ) and make sure you have plenty air flow going to it. Just my thoughts, Charles