If the bolt isn't broken before you remove the exhaust manifold there is always a good chance it will break during the removal process. Replaced my manifold (good old TJ stock cast manifold crack) and broke 2 bolts. You have the right idea on the fix for the broken bolt. Take a close look though at the exhaust manifold once you get it off. Don't be surprised if some of the leak is from a crack right at the center dump tube. If it's not cracked I would still replace it with one of the new and improved ones anyway with the relief coils at each end (cyl. 1 & 6). Do a search, I got mine for around $125.00.