To ease your mind on the box I would remove it and bench test it. Yes you can make some adjustment on the car but the best way is with it in a vise with a 0-30 in/lb dial wrench. You really need it to be 0-30 dial too, the clickers, beam, or other torque wrenches wont show you anything.
With it in the vise, without attempting any adjustment, to the end of travel and paint mark it on the input and seal. Then go to the other end and do the same, split the difference and with a late 69 to 82 box the D-Flat should be at 12 o'clock. Now you can use the TW and see how much lash drag it has going from end to end in both directions. At the center, called high center, it should read the highest reading. The needle should not jump all over the place, a little is ok with a used box but if it was rebuilt it should be smooth.
I have repaired a lot of "rebuilt" vette boxes, from vette shops, well known vette catalog houses, local dealerships, hot rod shops, etc. They were ALL poorly done and many were not even real rebuilds, just paint and grease. The dial-in settings are all over the place and many need new gears, which I think I have the stock on. Corvette Steering in SC was the supplier of box parts and new gears to most of the vendors, Tom died 3-4 years ago and I have his parts. NOT ONE rebuilder ever called for a new set of gears for their customers rebuild. Some have good used stock but many are doing unsafe things rebuilding these boxes. I am not saying that is what you're dealing with but I wouldn't be surprised to find flipped worm nuts or welded teeth either.
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