Thing is, even if the box moves around it doesn't stop the car from steering. It just manifests as play in the steering, so even if the mounting holes in the steering box are larger than the bolts in the K the box would just move back and forth a pretty small amount.
The steering mount on my K frame broke free of about half its welds, the entire steering box would move back and forth a good 1/2" or so with all the flex in the broken mount. The car would still steer, although there was a fair amount of play in the steering and responsiveness.
The issue the OP is having is probably not related at all to how the steering box is mounted unless something is directly interfering with the pitman or the coupler isn't working, which could be a result of modifications used to fit his previous non-factory box. Beyond that, it's more likely there's a hydraulic issue, which could be from the install or again modifications from the last box.
But we don't have any idea what was done to his car to fit the last box.