If a stud on the manifold breaks, it's not as bad as fixing a stud in the head. You can drill right through the manifold to replace them. The flange is exposed on the top side, so you can drill it out and re-tap for M10, or use any bolt that fits the hole. I'm not recommending that a through-bolt is necessarily the best repair, but it will work, and I've seen cars with that fix for a long time.

However, in my experience, apply PB Blaster over the course of a few days before attempting the repair, and the nuts will most likely come off OK. Careful with the rear one, it's a bit more of a challenge to get at. Expect to have to apply a pretty strong force to break these loose. Original nuts are 16mm. Replacements could be 15, 16, or 17. Get new copper M10 nuts from FCP Groton or another supplier before starting this job.

Good luck with it!
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