I tried adjusting my box with just the top nut & screw and had no luck. I've got an old manual that says the "worm bearing preload" adjustment should be done first. Where the steering column shaft enters the box there is a large "lock nut" you need to loosen first then underneath that there is a "worm bearing adjuster" you can put a wrench on & tighten. Then you adjust the "sector and ball nut backlash" with the nut/screw. I'll find my manual and post the steps later tonight if you'd like.
It worked perfectly on our 66.


Dave
66 GMC- father /son project with the "GMC KID"
36 Chevy PU- 4x4 lifetime project
64 Skylark- wife says its hers