Hello,
316, always the best grade of 316 you can get. If, for some awful reason, the chainplate itself is some other alloy, and you won't/can't replace it, at least your bolts will be good. No significant danger of galvanic action, as the difference in potential will be teensy.
As for head shape, as suggested either modify the bolt heads or find better-shaped ones off-the-shelf; sharp bolt heads on the outside of a hull are going to scrape someone, some day.
Fair leads,
Brion Toss
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