Grade 8 bolts with hardened washers.You have a choice of using regular nuts, nylock nuts(not good near high temp,i.e.ehaust),stover nuts are self locking type that donot like to be removed let alone back off.they can be tightened but are not designed to be removed.
I use 3/8 and 7/16 grade-8 bolts and hardened washers on my 72 3/4 4x4 at the tranny/transfer case crossmember.works well.

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