727 cooling line help needed!

There are a lot of pros and cons on this guys, and the pan has pretty much always been the spot for a temp sensor because the fluid returns to there after cooling and gives you a real time temperature reading of the fluid being used.
On the other hand being aluminum, the case is pretty darn close to that same temp since the fluid just got circulated all through it anyway.

A con for the pan is that it's a pia to put it in the pan and do a decent job of it where you don't have leaks.
A con for the exiting cooler line is as stated above in that you will get a higher than actual reading of the fluid that is running the trans.

Most any of the pressure ports would give a midrange temp reading, and I would personally use the accumulator test port because of the constant amount of fluid there during normal operation.
Just my 02