Is the control valve attached to the drag link to the appropriate depth? Generally, you center the steering box, thread the valve to the link until just a few threads are showing or as close to that as you can where you don't have to move the pitman arm very much. It should go without saying that if the box isn't centered and you don't fully thread the valve that you will run out of steering in one direction (sorry, can't recall which direction). Most of the time, it's not a very precise operation and the difference is within the design of the box and the tie rod adjustment.
As to the hard steering; Are you double damn sure you have the hoses from the valve to the cylinder routed correctly? Normally, you'll get a wild steering wheel when they're backward, but it's possible you just end up fighting hydraulic pressure. There is a bias adjustment on the control valve. To center the valve, you remove the cap on the outside end, remove the ram rod from the frame bracket, start up the engine and observe the movement of the rod. If the control valve is properly adjusted,the rod will not move. If it drifts out or in, you have to adjust the valve. It doesn't take much turning of the nut to affect the valve bias.
The power steering system is made by Saginaw...yeah, a GM system used on Novas and 62-82 Corvettes and others. The guy who engineered that system is Jim Shea. He's still around and has published just about an entire library of how to work on the system and every engineering specification. Google him.
