moving coolant temp sender

I'm in the process of upgrading my cooling system after a recent radiator failure. As part of my upgrade I'm moving the temp sender from it's stock position in the primary coolant tank. I purchased a new sender on Ebay for $8, so I plan to simply detach the wire from the original sender and leave that unit in place in the tank. Then I'll remove the existing plug from below the thermostat, place the new sender and attach the wire. Seems simple enough.

I found the following link helpful: Banzai Runner Pantera Thermostat Replacement

As I've gone to remove the plug I find that it is quite determined to keep it's place. I have no reason to believe the plug has been manipulated since the motor was built in 1973. I've applied penetrating oil and am letting it sit overnight. Perhaps I just need to be patient with an old, stuck bolt but I want to be sure I'm on the right track.

Below is a photo of my block. Note the plug has a square head on it. Access is difficult but I'm just able to fit a box-end wrench over top and get a good position of leverage. So far it ain't budging. Am I on the right track and just need to persist?