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The right way (IMHO) to run it is tranny to ext cooler, thru the radiator, back to the tranny.
This method cools the fluid, then runs it thru the cold side of the radiator (where is can pick up some heat if needed in cold weather) and reduces the heat load on the engine cooling system...
As stated above, do not do this in cold weather as too cold fluid is almost as harmful as too warm.
As for the opening size, don't change it unless you seal the edges so that the air cannot go around the radiator. it is stepped like that due to the airflow characteristics of the rad/core support.
Installed 180 stat,external cooler with tranny to ext. cooler then to radiator,made opening in front of radiator same size as radiator--good ole sawzall--engine runs cooler than every on 102 degree day! Thanks everyone for great input!