https://www.flickr.com/photos/advance-design_parts_co/sets/72157632210242370/ The above is the best steering manual out there.
I would unhook the drag link and center the steering box. This will confirm the location of the steering wheel, Center steering wheel if it is not centered to the gear box. Wrap masking take around column and base of steering wheel, draw a line across both for a true center indicator. Next turn the wheels all the way left then all the way right, eyeball the angle and see if one way looks different then the other, adjust stops if they are way off. Set the wheels straight ahead and see if the drag link will drop into place. It should fall in to place.
If the leaf spring bushings are worn, or the leaf spring center pin is worn, the axle can shift forward or backwards, this then creates a problem with the draglink length. If the distance is not what the factor set it to, the steering will be off center, causing just the problems you are having.
To correct this on my truck with a newer axle, I had to build an adjustable drag link to dial in the straight ahead position. I never did figure out why the axle wasn't centered, but the adjustable drag link solved all my problems.
Joe