For those debating where to place the sensor, I've done this on two different transmissions (4L60E and 4L80E) by using the pressure test port as OP did. Took the truck out and beat it mercilessly, then checked all over using an IR thermometer. Reading on the gauge was almost spot-on the hottest temperature I could find anywhere on the transmission - all around the pan, case, torque converter, etc. I have 100% confidence in using the test port as an accurate measuring point, and prefer it greatly over risking introducing another potential sudden-massive-leak point by splicing into a cooling line. YMMV.
Blue95 - you can get SEM at most auto paint supply stores. Be sure to get the adhesion promoter too, read and follow the instructions to the letter for the promoter and the dye. Very important to have some grease-removing cleaner. SEM suggests TSP (trisodium phosphate) which can be had at many (but not all) hardware stores, or you can also find TSP substitute (calcium carbonate.) I guess for something as small as a gauge pod it may not matter much; I did a whole interior with the stuff so I wanted it to be durable.
When I did my 1996 3500 it was just a wrecker, not a play truck - so I didn't care much how the install looked. I already had a gauge and switch for my helper air bags and a switch for my load lights, so I used an Autometer 3 gauge panel:
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Did a much cleaner install in the 1994 2500LD using an AMI GMP333 stereo delete pocket:
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Richard