1) unhook linkage or pitman arm from gear box
2) adjust gear box per manual ( https://www.flickr.com/photos/advance-design_parts_co/sets/72157632210242370/
) wrong year I know, but adjustments are the same
3) center gear box, full left to right, back to 1/2 of total turns. You should feel the tight center spot. If not, the gears are worn or the adjustment is still off,
4) center steering wheel on shaft, recheck with total turns then half to center,
5) reattach the linkage.
6) check toe-in adjustment, tape measure and a helper is about all you need, also check air pressure in tires
7) go for test drive, if steering wheel is still off, rotated front tires left to right to rule out tire wear.
Move on to suspension if still not happy with setting,
8) check front and rear bushings on the front leaf springs, they must be tight and in good working order,
9) check center pin on the leaf spring to axle alignment hole, will need to loosen u-bolts ( best to replace u-bolt at this point ),
10) Check front to rear wheelbase on each side to be sure rear axle is aligned to chassis and front axle,
11) check frame for square, measure diagonal from corner to corner
12) measure both axle locations to frame, find a point on each frame rail that is in same location ( mounting hole, bolt hole, rivet ) to be sure axles are where they are suppose to be
If the steering wheel is still offset after all this checking, an adjustable draglink may be needed. My own truck checked out good, put I never could get the wheel straight while driving straight. An adjustable draglink let me dial it dead on straight on a true flat road, it now drives as good as my '03 Toyota. Just a very slight turn on the draglink center bar changes the steering wheel location quite a bit. It doesn't take much error in the steering and suspension to mess up the steering wheel location.
Just don't move the steering to correct the problem, the truck will drive much better with everything centered and adjusted as GM wanted it to be.
I used tie-rod ends and custom made threaded center bar for my draglink. If you have an old center bar ( long bar tie-rods bolt to ) cut off the right hand threaded end, shorten bar and rethread with correct die, you now have an adjustable draglink without much money spent. New, straight shank tie-rod ends bolt right in to the pitman arm and steering arm.