I see the plug you are referring to and can't tell you for sure if that is a good location. In a nutshell, the sensor needs to be immersed in ATF. If that location threads up and doesn't interfere with any linkage, internal or external, and is below the fluid level, go for it. If it were me, with the pan off, I'd look at the end of the dipstick to see what the level of the fluid is inside the case. If that doesn't work, there are adapters to fit the sender to the pan. Do not tap the pan itself, as it is way to thin to hold the sensor. I just checked Summit racings site and saw that AutoMeter has a manifold for a sender you can splice into the cooling lines for $40. If you go this route, make sure it is the line that has not been cooled yet. To determine 100% which way the fluid moves, disconnect a line and bump the key. It's messy, but effective. To me, this would be the easy way out because you won't be drilling about a 5/8" hole in your pan, which if not done correctly could lead to a leak. Not correctly includes drilling a big hole in sheet metal with a regular drill bit. The hole will not be round and if it catches a burr on exit it will probably twist and warp the pan enough to cause @#$%$%$& leaks. Find an electrician or a sheet metal man that will let you borrow a Unibit, aka a step drill, for the hole. They cost about $40, so if you have to buy one, may as well have gotten the AutoMeter setup and saved the headaches and the expense of a fluid change. Good luck,
Phillip