I have removed at least 100 International Harvester exhaust manifolds. I have never had a exhaust bolt break. I was talking with another mechanic that is used to working on all different brands of engine and he was worried about the bolts breaking off in the engine block. Fortunately this does not happen to IH engines because IH used copper clad bolts that somehow are able to prevent the bolt from bonding to the block. It is hard to tell that they are copper clad bolts , because they are only shinny copper colored when brand new. The bolts turn black the first time that they are ran up to temperature.
So word to the wise, if you remove an exhaust manifold make sure that you replace the bolts with these special copper clad bolts. They can be found here. 