Grade 8 bolts in that application are over kill. Save a buck or 2 and go with grade 5.
The extra cost of grade 8 bolts is minimal compared to the peace of mind one gets when securing things with them. Besides someone asked where to get them. Besides Lowes and Home Depot I guess. Aco, Ace and Tru Value all sell Hillman fasteners. Fastenal is ridiculous in that you have to buy a whole box of bolts...at least the one here is like that.
Old but having fun
Real Americans drive Real American!