Get the shock back off and do what you have to, to get the broken stud off. I used a 4-1/2" grinding wheel, didn't take too long. If you are worried about fire, then make sure you have a hose handy. The stud is actually pretty soft for steel, you could probably use a hacksaw.
Once you are flush, some people can punch the last little part out. I had to drill a 1/2" hole. (Don't buy the drill bit at Orielly's, I did and it was dull right out of the box).
Get the shock stud kit... orielly's should have it on the hangers. You will see that the short threaded end of the kit goes through the radius arm, the hole you just made, and the long end goes through the shock. There will be a nut in the kit for the short end. I have found that this nut can come loose and disappear. If I were to do this again, I would buy a locknut with the nylon insert and use that instead of the nut they provide.