the right pitman arm

Also, I found PB blaster is better at rust than WD-40. As for that nut, it is on there. Try to use a socket with no adapter or extensions. Those loose some of the impacts strength. The arm may or may not fall off either. A few taps with the hammer on the pitman arm itself can pop it loose from the sector shaft. They sell a tool for removal at auto part stores. Some even allow you to rent it very inexpensive.
Good luck!
Syleng1