If it were mine I would mig/tig weld a nut onto the end of the remaining stud - let it cool until it's no longer red hot - then give it a gentle turn both ways. If you get some movement, give it the penetrating fluid and carefully turn it both ways until it seems like it will take more torque to come out. If the nut breaks off, weld on another one.
I would put the ground cable as close to the weld as possible and unplug the wiring harness from the engine.
I would put the ground cable as close to the weld as possible and unplug the wiring harness from the engine.
