I've snapped off more than a couple in my B18 and B20 blocks. Tightening slightly is surprisingly effective.
Given my past issues with seized brake line fittings I've fallen in love with the propane torch and water spray approach. It always cracks with a satisfying sound. Only issue is that you can't help but screw up the annealing process originally applied to the metal. It may be particularly risky with an exhaust manifold as it will only get more brittle if you cool it suddenly.
personally I'd take the risk, if you heat up the manifold to red, be sure to reheat and let it cool slowly. Use a focused spray of water on the nut/stud and be sure to replace all the studs with new. Drilling out snapped studs is a very trying process.
Usually, IME, you end up backing the stud out of the head with a rusty bolt in place. There are far worse outcomes.
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