I found grade 8 M6 bolts at ACE hardware believe it or not. The guy there explained it to me. It's not colored like a grade 8. There are no lines on the head like a grade 8. But it actually says "8" right on the head. It's silver in color and not too expensive. But I went a step further and bought M6 metric bolts high polished chrome in the specialty rack but those puppies are $2.50 EACH!! They haven't broken on mine yet though.
There is a good chance the studs you have on there is grade 2. Using a bolt isn't much of a hassle, Get one long enough and get the nuts and washers in there first,tighten down the bolt then lock the nut down. And look for that number 8 on the head.

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