Is the only adjustment to take up the slack in the power steering box the screw in adjuster with the lock nut on the top of the power steering box? Or is it like a Oldsmobile and have another adjustment on the bottom of the box?

Typically, at least on the later GM steering gears (power and manual), there are two adjustments . . . the "upper" adjustment with the locknut and the "input side" or "adjuster plug" adjustment. As the steering shaft (input side, from the steering column) turns, it has a "reaction" with the sector gear and will try to push away from the other gear. Therefore, this "in-n-out" motion is controlled by the adjuster plug (usually has a spanner-type nut PLUS the input shaft seal) adjustment. The "upper" adjustment is related to the mesh of the input shaft and the sector gear (attached to the output or pitman shaft). Rather than being "too tight", it is best to have it just a smidgen on the loose side, so the self-return will work (without the driver having to fully return the steering wheel themselves after making a turn). As mentioned, this is how the later ones work and I suspect the earlier ones can be similar.

I do know that you can adjust the "upper" adjustment all day long, but unless the input/adjuster plug adjusment is accurate (keeping the input shaft's in-n-out movement to a minimum), you can have tight steering (higher effort with no caster return action, if any) that is still loose. BOTH adjustments need to keep things "in spec" for everything to work right.

Just some thougths,
NTX5467