1/4'' shank/ 1/4'' ''double cut'' carbide burr and a right angle die grinder, keep your rpms low on the die grinder and center it on the bolt shaft that you can see, if you drilled it pretty straight, this should be easy. lighty push the carbide burr into the bolt and let it chew its way through and be very careful to keep it in the center. keep some WD-40 or other light viscosity oil around and keep the carbid burr lubricated so it doesn't load up with chips. grind all the way down through the bolt until it falls on through, you should have a hollow cylinder left in there with threads on the outside, now grind out as much as you can without getting into the alumin threads, after that get a sharp center punch and try to fold the threads in on themselves, it should all come out in pieces like half circles. keep at it and grind out some more steel when necessary. if your careful you will have perfectly good threads left in there