Hi. I just picked up an 83 300CD w/ 263k on it and catching up on all the routine maintenance, starting with the valve adjustment. I'm stuck on the intake valve of cylinder 1.

The valve is tight but I can't adjust it because I can't loosen the locking nut. I'm using bent wrenches, confirmed that it doesn't interfere with the adjustment nut, and turning ccw. I applied a lot of force, enough to be concerned about stripping the nut. Any advice?