Hey Treiberg,
Diesel 911 has some good points. Use a flat allen socket. Heat can't hurt. I'll bet Al will expand quicker than steel. Diesel is correct, heat the sucker and then smack it with a slightly undersized allen. Your idea about the heat gun may work but you will not generate the searing heat of propane. Have on hand a spray bottle filled with water.

If you are getting a good size breaker bar and still you feel it slipping either you have not fully inserted the allen (junk in the bolt head) or the allen itself is rounded or the socket is of poor quality (no dig intended). I noticed the same thing on the one of the 6mm allen bolts. It was slipping like you describe. I took a good look at the end of allen and noticed it was only inserted about half way. Gently tap in and have at it. It will break free.

If all else fails and you have access to a good indy shop take in and have them break the bolts. May cost you a couple of bagels.

Do you see allot of corrosion up in the bolt head area?

Good luck,
Zill.