I just replace my power steering gear box in a 68 Fury with a new Cardone. Now, on taking a 90 degree corner, when I let go of the steering wheel, it does not return to near center as it did before. The wheel (and car) stays in a hard (left or right) turn.

Is the valve properly centered? (Does the steering wheel have any tendency to turn itself, especially with the front wheels off the ground)?

jack the front wheels off of the ground. center the wheels. start/idle it. wheels should not move/pull to one side.

it is unlikely that the valve is centered. you will need to do as robert says and try it with the front wheels in the air, and then adjust as needed.
don't stick your arm through the steering wheel when starting.

It doesn't sound like it pulls to either side just doesn't return. That would lead me to believe the box is adjusted too tight. The nut on the body with the screw in the center is the adjustment for that. Loosen the nut and give the back the screw out a little at a time until the steering returns to center like you want.

I had this on a number of different Mopars. The steering wheel was too tight to the column. I pulled the steering wheel and tightened it only enough to take out any play to the column. It is a jamb nut so no worries about it coming loose.

I suppose it could be, but I have had the valve adjusted wrong as well where it won't allow return.

It could also be alignment etc..
but if all he changed was the box, then that is where I would start.