If you have the clearance, it would probably be easier to use the bolt extractors...

http://www.irwin.com/irwin/consumer/jhtml/detail.jhtml?prodId=IrwinProd100514

Yes, it's round on the outside, however the bolt extractors will bite into about anything.

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A.S.E Tech A1,A6,A7,A8 & MVAC 609 + EPA 608

Unless stated otherwise, any question I ask is about my greymarket 1985 380SEL.