Turning the steering wheel from lock-to-lock, and then going half-way in between, is the START of finding center-point on the gear. If the vehicle alignment is far enough off, you'll get a false result if the wheels hit the frame, control arm, or whatever before the steering gear gets to the true end-of-travel position.
You need to verify that the stub shaft--the shaft that the rag joint bolts to--has the flat on the shaft facing upward, parallel with the top cover. (GM calls it the "side cover", but it's on the top surface of the gear when installed in the vehicle.) Meanwhile, the Master Spline on the Pitman shaft is in-line with the adjuster screw on the top cover. You need BOTH to be in proper position, or the steering gear is built improperly.
Diagram for this is in the service manual. On my '97 C/K manual, text is on page 3B1A-4, and the diagrams are on the next page, 3B1A-5.
The center-point of the steering gear is the tightest, least-free-play point of the gear. If the vehicle is going straight down the road, the gear MUST be on the center point, or the steering feels "sloppy". This is really easy to get wrong, if the tie rod ends aren't adjusted properly, or if the gear is built wrong. (Lots of complaints about "sloppy" steering...)
If this vehicle has been "aligned" multiple times, and the morons allowed it out-the-door with the steering wheel upside-down, YOU NEED A DIFFERENT ALIGNMENT SHOP.