It might be too 'strong'. As the hardness and grade go up, so does the brittleness (you wouldn't want a very hard diamond bolt in there, would you?). Most receiver hitch manf supply and specify Grade 5 bolts. Dexter does spec the torque for your kit's 1/2" at 70-90 ft-lbs.
It might be a moot point, however, as I think the correct hardware comes with the A/P -165 hanger kit. You could call Dexter to be sure about that. Give us some feedback.