I'm trying to get 2 bolts out of an Ibiza 1.2 sump. The insides of them have been destroyed, the outsides are sloped so I can't get a grip on them. They're on the RHS so there's a kind of thin cast metal flange that looks like it will break if it's hit ruling out tapping a torx in. Question is would welding a bolt on to the bolts do any damage to the cast metal ?