![Sad :(](./images/smilies/icon_sad.gif)
Welding a nut on MAY help, but I think you are going to need more heat than that to break it loose. That knuckle is a big heat sink and doesn't heat up fast.
one nice thing though.... you can drill all the way thru it and not worry about what is behind it.
![2 cents :2cents:](./images/smilies/icon_2cents.gif)
good luck!