I'd definitely do what he said, soak em good with penetrant for a day or so. Also see what kind of condition they are in. If they aren't covered with scale rust or anything, I'd say you should be able to get them out ok. I've put headers on a lot of vehicles, and I have never broken any yet. They are usually pretty heavy grade bolts. One trick I have used once or twice taught to me by another mechanic is if you get one that seems stuck and wont budge, rather than trying to power it out and possibly breaking it, start the engine and let it run for a minute, enough to get the block slightly warm (not hot), and then shut it down and try to remove the bolt again with the block a bit warm. I'm not sure exactly why this works, but both times I did it, the stuck bolt came out without breaking.
Dave
