I'll answer my own question... There is no easy access to all the nuts, but that's irrelevant, because they are glassed over from inside. This is good as water should never infiltrate the hull if/when a bolt fails. The fact they are carriage bolts makes this job tricky. My solution was to grid the heads off, drill out the remaining square part of the bolt, then extract the remaining threads using either an extractor or welding a bolt on, depending how stubborn. To replace the carriage bolt I tack welded a hex nut on, applied sealant and tightened, then broke of the hex nut, and ground the welds off with a flapper disc.
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