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Easiest way is to weld a bung in the pan for a temp sensor, or put a T on the cooler line coming from the tranny to the cooler and put the sensor there. You want the temp as cool as you can get it presuming its not sub zero outside, as far under 200 as you can do. I run an extruded aluminum cooler by itself under the car and do not use the cooler in the radiator at all
I use an Autometer trans temp gauge. I bought a drain plug kit, which required drilling a hole in the pan, but no welding. The the sensor for the temp gauge screwed in place. The drain plug kit was less than 10 bucks and I bought it at Summit, along with the gauge. This way, there was no welding, just one hole to drill, and I still get the benefits of my magnetic drain plug. I drilled the hole and mounted it on the front side of the pan and not the bottom, to lesson the chances of ripping the wires loose.