I prefer the sender be in the pressure test port. That is on the driver's side of the trans, just above the pan, towards the front of the trans. There is a black hex head plug there from the factory.

Adding a port to the pan adds a potential leak point, but more importantly, reading the temperature down low in the pan will show you the coolest spot in the entire trans. It's where the cooled fluid that has just returned from the cooler is located.

 

Good point about the cooler return. Do you happen to know why Ford chose to put the temperature sensor in there then? I realize the sensor is actually on the valve body instead of submerged in the pan... is there that much of a difference between those locations?