This reminds me of something; there's a metal plate between the underbody and frame at the top of the shock tower, heads up if you get that cup thing out and start drilling for a timesert. I'll get a pic from below so you can see what to expect. (You can see the metal plate and the area where the nut is by looking into the wheel well.) You might be able to reach that nut with a long screwdriver & hold it while tightening the body bolt.
*edit: Here's a pic from my last car before restoration, it looks possible to work through that opening to tighten or replace the nut.
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