Soak them good in some kind of penetrating oil, pb blaster or similar. Allow to set over night before you attempt to loosen them. You should have no problem with them breaking off, I have yet to break off a header bolt. The biggest problem I encounter are the heads of the bolt being rusted so bad that a standard size socket wont work , so I have had to go to a easy out type of socket designed to grip stripped bolt heads.
Take your time and don't force them, if you encounter one that feels like it wont budge use a propane torch and heat the head of it up, then quench it with pb blaster , do this a few times and it will eventually let loose.