Looks like the head of the allen bolt sticks out a good bit from the manifold, I would take a dremel or cutoff wheel and cut a slot in both the bolt and the easy out, then use a good heavy flat bladed screwdriver (I have a big old snap on screwdriver with a hex on it that has come in really handy for things like this before) or possibly one of the impact screwdrivers to back it out.
You can also weld a nut to it as mentioned above, sometimes the heat from the welder or the cutoff wheel, and the subsequent cooling down can free up stuck bolts like that.