Too late now, but for future reference....
Grab yourself a large diameter brass drift, around 3/4 in diameter and about 6" in length. Place the drift against the head of the allen bolt and smack it really hard with a substantial hammer...you may have to hit it several times...
Then using a hammer driven impact with a hardened allen bit of the correct size, first tighten the bolt with one smack, then loosen the bolt....
Used this technique for 30 years...have never had one not come out...
