From looking at the 67-72 site, your truck is set up with a much more complicated steering system then the old trucks.
Your pitman arm is hooked to the center section of the steering arms. Drivers side is held up by the steering box and pitman arm, passenger side is held up by the idler arm. On each end of the center section are two tierods (four total). One inner and one outer with a adjustable sleeve between them.
You still need to unhook the pitman arm from the gear box and make sure the wheel is mounted dead center. Go lock to lock and split the difference.
The pitman arm is most likely keyed so it goes on only one way. If you really clean the arm and shaft, you might find some very small index lines. They may or may not be there, the arm will go in the straight ahead position or straight back depending on the location of the gear box.
To center the tires, the tierods will need to be adjusted. Since there are inner and outers, you can adjust each wheel separate. You still need to maintain the toe-in adjustment, so if you turn the left wheel out, you need to turn the right wheel in or which ever way get you where you need to be.
If you can turn more one way then the other, two things can cause this. One, you are hitting the axle stops with the spindle (this would be mis-adjusted tierods) or TWO, you are hitting the internal gear box stops (mismatched parts). So turn the wheel as far as you can (have the tires off the ground so it turns easy) and look at each axle stop. Axle stops are adjustable bolts on the back side of each spindle that hit the axle and stop the spindle from going to far. Internal gear box stops keep the gear box from killing itself.
Let us know what you find.
