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.
Yes we had loosened it about 1.5 full turns and then held it in place with a 5/8 wrench wedged into the radiator support and the screw is seized into the nut itself and nothing was getting it loose