I'd say either would work fine, more than likely even Gr8 would be an improvement over original in strength. I'd go with fine thread and permanent LocTite. Whether bolt or rivet, most joints on a frame are primarily designed to work in shear, not tension, so the head size is not likely to be critical.
'69 1/2T stepside, Sea Foam Green, 250 L6 w/ 3 on the tree, not an option on it, in the family since new.
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