The same thing happened when I changed out my son's 03 H pipe. Even after hitting them with pb blaster and letting them sit, then heating the nuts. Snapped like it was their job. They are threaded into the manifold but even when I cherry hotted the manifold flange with a torch they barely wanted to turn and there isn't much room to get on them with anything.
I just cut them flush and drilled through them. (They snap easy but are a pain to drill) Used longer bolts and put nuts behind the flange.
The passenger side came off fine thankfully since they are harder to get to. Before trying those spray the heck out of them with a good penetrating oil and let them sit for an hour or 2 to give yourself the best chance of getting them off.