Been there, I feel your pain.
It's going to be *much* easier to remove the knuckle and tackle this off the X5.
I centerpunched the stub, EXACTLY in the center, then used a set of left handed bits to drill increasingly larger holes in the stub; I was able to eventually back the remnant out with an extractor and a pick. Cleaned up the threads with a tap.
I'm a decent welder, and there's no way I could weld anything to the stub left in mine, it sheared at an angle and half of it was below the surface of the drilling in the knuckle.
