For reference this is a 5/8 lock nut with an Allen screw inside. It was so seized up, we let it soak for 3 days with antiseize, tried head and freeze method, and nothing helped. Still ended up with a stripped screw. I’m at a loss at this point and not sure I feel comfortable grinding a notch into it.
The screw doesn’t have enough exposed thread to get a good grip with the vice grips. Tried out needle nose grips and it was just spinning on the threads still. The screw isn’t replaceable and I wouldn’t if I wanted to, but the nut is replaceable so I might try splitting it. I would be very worried about damaging that screw though as it would mean a new steering gearbox if I did.