I'd put it in the test port, especially if your going to get a pan later. That way you dont create a week point in your cooler lines for a possible leak later, even though the pressure is minute.
Another reason I dont like it in the cooler line is, if you put it before the cooler your just reading torque converter temp, if after the cooler the oil temp will be lower than the actual sump temp for the most part.
A sender in the pan you get a reading of how hot the oil is that is feeding your transmissions parts, main thing being seals/o-rings and fluid that heat effects most. In the pan the temp will be hotter than the cooler line because of heat created from the pump feeding engaged clutches/lube circuits plus all the friction from gears meshes under load and gear changes, all this oil drains back to the pan and recirculated uncooled.
If your good with a welder or have a friend, just cut a steel 1/8" pipe union in half and weld one half to the pan, drill and your set. I've even done them on the truck before(do it when its full of fluid, so its submerged to help cool it and no oxygen for a fire inside), have your sender ready, let the fluid wash the shavings out when you drill, stick the sender in and your done.
Here's the best pic I have, the temp sender is in the upper right, the drain plug(1/4" pipe) was made the same way also. You can also see the sender in the pressure port(upper center) for my pressure gauge.
