The interference is in the rockers. You can pull them off the shaft and wrap them with a hammer to tighten the adjuster screw. And yes, the threads can be worn through 50 years of adjusting but the adjuster threads are softer than the rocker threads.
If you add locknuts you need to spot face the contact surface parallel to the adjuster nut. Like I said, there is info here on FABO bout that. A machine shop can do that.
