You have to be very careful adjusting this or you will wreck the gears for sure.
All the vette boxes I built that needed gears 80% of them were bad from inproper adjustment.
Play in the steering box comes from the following areas:
1- lash adjustment loose or overadjusted
2- lash screw gap- sloppy from day 1 box needs to be torn down to correct
3- sideplay in the bushings- if stock they are about .010, rebuild kit bushings will drop them to .005". I custom fit mine to .002-.0025"
4- worn rag joint
5 worn suspension parts
If you adjust on the car be sure it is not too tight on center. I can't stress this enough, gears cost well over $200 these days, $245 the last set I installed a few weeks ago.