As far as diameter and thread, as long as the nut and bolt match, does it really matter? I'm pretty sure that everything DF supplies that doesn't screw into a Cobalt part is SAE, not metric and I'm pretty sure they supplied 1/4-20 bolts/nuts. As far as length, there is plenty of room behind the panel if you go a little long.
And 6mm bolts are just slightly smaller in diameter than 1/4".