OP should have used a cobalt drill bit to remove the bolt head. eg. Irwin 3016028 Cobalt Drill Bit, 7/16" $6.95.
For the benefit of anyone else with a similar stuck bolt, this happens with steel-aluminum corrosion. One way to break the corrosion: You will be using a six-point socket, not a 12-point. Before you bung up the bolt head, try re-torquing down all the head bolts completely and then over-tighten the stuck bolt by just a bit. If it turns even the slightest, you will have broken the corrosion and then you can alternately loosen and tighten the bolt and it should come out a little at a time. If that doesn't work, see if there is sufficient space to put a punch against the bolt head and hit it with a BFH. If that doesn't work, and the bolt head is not recessed too much, try to put a pipe wrench on the bolt and use a big cheater. Pipe wrenches don't care if the bolt head is bunged up.