I wouldn't do a grade 8. The properties of the bolt aren't ideal. They are stronger but, you get a strong enough lateral force grade 8 bolts will just sheer and break. Grade 5 bolts will bend and distort before breaking.For future reference... a 3/8" socket cap bolt with a grade 8 nylock nut works better than any leaf spring center pin. AND they rated bolts, where as the center pin bolts are not.
Any Ace hardware should have them.
I have seen this once before..