I put mine in the test port behind the shift linkage. You won't be able to see it but you can feel it and get a 1/2" wrench on it. Fair warning, mine was tight as hell.
isn't the test port a low flow section, so it doesn't give an accurate reading? I would think the pan would give the most accurate overall. they have the spot where a drain plug should be anyway. I have seen kits for putting them in there and you just unscrew them to drain the fluid without all that fun mess!
The test port seems to work fine for me. While it may be a low flow section, it seems to give me the response that I want. Normal cruise temperature on a hot summer day is around 200*, loaded is about 220* and a cool temp. cruise is about 170*. I do remember a couple of years ago, I was hauling heavy in the summer at about 220* and ran right into a rain storm. Within a minute or 2 the temp had dropped to about 170*. Needless to say, it responds well in the test port. Don't get me wrong, I think the pan is the best location. I just didn't have the adapter or was inclined to go get one, drop the pan and change the fluid; bottom line is I was lazy that morning.