If you are using the 3" bolts for the grate supports, a zinc coated bolt will be fine. Zinc melts at ~786°F and fumes at ~1665°F. If you have those temps at grate level, you have greater concerns than zinc exposure!
Now, if the bolts will be used at the coal basket level, find some stainless...
Now, for the naysayers, the total surface areas of the bolts will not provide enough zinc exposure to cause any long term health effects at normal, or even elevated temperatures on a pit this size. Now, this is only for the 6-10 bolts needed for the grate supports, not for sheet metal, strapping etc...