The only thing I can recommend is that you get the proper T-60 wrench in a form that allows you to get it as snugly into the head as you can, and then to wedge it tightly between head and body so that it can't cam out. You can hope that the allen wrench didn't get all the way into the outer points of the Torx star, and left enough there. When I did mine, I welded the body of the socket to a short flat wrench, and used a big flat prybar to jam it against the bolt head. A big cheater on the handle of the wrench, and it finally popped. Depending on the shapes of tools, this may be a two person job.
Make sure the socket of the bolt is well cleaned out with no burrs or junk in it, because it's pretty shallow at best. If you can really jam that wrench in there and keep it straight, you may still have a chance. Of course as soon as it begins to turn you'll have to unwedge it to allow it to come out, but if you get any movement at all you've won the battle, and the rest is just a matter of patience.
Unfortunately, the bolt is so hard, and there's so little protruding, that it's not possible to get vise grips or a pipe wrench on it. The wrench simply won't bite. One pretty drastic solution to this if the head is really mugged would be to saw, file or grind flats onto the little bit that does protrude, and see if you can get a big wrench on it (note, this is just an idea, never actually tried!). Beyond that I think you're getting into "pull-the-engine" territory. With the engine out, you could weld something to the bolt, or grind the entire flange off it and slip the adapter off, but of course this would also mean you'd have to find a new bolt.
