Had the same problem but with an easy-out already stuck in the bolt from someone trying to remove it. Spent lots of time messing with it. I finally got the easy-out removed and then just drilled out the remaining portion of the bolt. This of course damaged the threads in the case. (Fill the drill bit with grease to catch the removed metal bits as you drill - careful as you get to the end not to damage the tensioner - once the hole is complete drain the oil and flush with about of pint of diesel oil. The remaining bits, if there are any, will not get past the oil filter.)
Finally figured out that you could tap 3/8 threads in the larger hole (near the outside of the case) in the case. So I bought a piece of 3/8" threaded rod and ground down the end to fit the small hole in the case. Threaded the large hole, cut the 3/8" to the proper length and added a locking nut.