See if you can drill flats into the bolt head so you can fit a really big flathead screwdriver tip into the bolt. Hopefully your really big screwdriver doesn't have a round shank so you can grip it with vice grips for more leverage.

As a last resort, drill off the head of the bolt then deal with the remaining stud when the injector retainer is out of the way.

It's been a while since I had the injectors out so make sure this is possible before you start.

Sixto
91 300SE
87 300SDL
83 300SD