most exhaust kits will come with new hardware.... usually the old stuff is rusted on and a pain to get off. I would suggest zapping all the bolts with some good rust eater a few hours before you tackle the install. This should make everything a whole lot easier.
If the kit did not come with new bolts, I would go down to your local autoparts store or hardware store and pick up some new fastners. Just insurance that something won't break down the road.