Reply to "moving coolant temp sender"

Sooner, that pipe plug was likely screwed tight into the block when it was hot. Some come out moderately hard, others are badly rusted in place.
First, take the alt. adjusting strap off to give yourself some wrenching room, then heat the plug end up with an oxyacetylene torch until the square is red-hot. The classic way is to then slap the end of a candle against the red-hot end to shock-cool it. Water from a spray bottle also works but is messy. Let it cool all the way to room temp and try your wrench again. Some then almost come out with fingers.
FWIW, the removed plug will fill the original hole in the water tank, but use 1 full turn of pipe tape on both the plug and the new sender before tightening them in. Too many turns of pipe tape on the new sender may electrically insulate it so it doesn't work at all. In this case, more is NOT better.