Put it on the out going line to the radiator near the radiator where you can see it and get to it easy. You will also be able too soot it with a infrared digital thermometer to see if its accurate.
Street cars using an external cooler should be run through the radiator first, then to the cooler and then back to the trans. It is more efficient to bring the fluid to radiator coolant temperature. The fluid will then cool more going through the cooler.
