Probably should inspect the remaining bolts at all other attach points too. Depending how they are oriented, they may have been seeing more load and perhaps loading in a direction or manner they were not designed to take because the AFT section was free to move up and down. If they look fine, retorque and go. If anything looks off, or if you see signs of movement I might replace them at the same time as a safety precaution.

Biggest bolt that will fit through the holes (5/8 maybe?), flat washer on each side, lock washer on the nut side. Crank them down hard, don't overthink it. I wouldn't bother with grade 8, the bolts that came with the hitch were probably grade 5 if they were even graded. Also, grade 8 are more likely to snap than stretch or bend, while grade 5 or lower are more likely to distort than fail suddenly