I don't know what kind of access you have to the bolt but I use a disc cutter (or dremel w/ fiberglass cutoff wheel) to cut a slot on top of the bolt. A normal screwdriver or ratchet w/ flat blade socket can be used to remove.

__________________
Rick

'85 380 SL (sold)
'85 Carrera Flatnose
'71 280 SL Signal Red/Cognac