there is not a chance I would use hardware store fasteners in an engine. If you are concerned about the bolts, just go ahead and get the stud kit from ARP. Grade 5 might actually be the correct bolt for these engines, it is strong, but will flex a little, whereas grade 8 is rather brittleish. but I don't know for sure what would have actually been specified. You said the one looks harder, just what does hard look like?
