Agree with the LH drill bit, probably starting with regular 1/16” (or about - just tiny) drill bit for a pilot hole. Stay as centered as possible, then something LH 5/32” or less, to prevent mucking the threads. Usually I have good luck with a LH bit, they bite quite a bit and the pilot hole usually induces a little heat close to the threads. Worst case scenario: you get a little off center and take a little bit of threads off the side of a non structural bolt hole. Re-tap. I have a bolt extractor kit, and I don’t think it has ever worked. I guess they’re made for un-seized bolts, that the head just fell off of

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Side note: I found an interesting way of dealing with stubborn bolts in decorative places many years ago. My pops had a seat back on his Harley Bad Boy that somebody had mounted with the strongest Loctite I’ve ever encountered. He’d already rounded the edges off a hex head bolt when he came over for help. We taped off and covered everything but the rounded bolt. Then tack-welded a nut on it (as most already know). But, the heat from the weld penetrated the bolt so well, that it backed right out afterward. So we did the same to the other three and it worked great. It’s a trick I learned and filed away for applying heat to a bolt in a pretty, painted area, where I would never use a heating torch. It’s worked on a couple other similar situations without fail, even when corrosion was the problem.