A couple of methods for removing fasteners are:

Use a dremel or disc cutter and cut a slot into the head of the fastner. This will allow you to use a (hopefully large) flat blade screwdriver to remove.

Weld a large nut onto the stripped head. The weld may have to be on the inside of the nut. You can then use a wrench or a socket to remove.

Use of heat (torch), PB Plaster and/or tapping the fastner will help things along when using the abovementioned methods.

__________________
Rick

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