Problem with X-pipe installation

Were you able to get the nut off? If you were, drop the pipe, and use a deep socket, and loosen the stud out of the header flange.

If the nut is still on, and you can't get it off, you will need to get a cut off wheel and cut it off.

Can you still get a grip on the nut? It's metric if I remember right, or a 3/4 (17mm). If you can, try loosening, tightening, loosening, etc etc until it backs off. If it's just plain stuck, or the nut is too stripped, you will need to cut it off.

Once you do, get a new stud from merles or somewhere.
Scott