I have found in a few instances working on various makes/models that the pittman arm does not have a dead spline and can be reassembled onto the steering box in any position.
An F150 I had gave me 4 choices of spline position to reassemble, of which only one position allowed the arm to go on the steering box shaft, which caused me the same problem you are experiencing.
But since we are talking Chevy, I have to agree with 89squareburb and would first verify the box was reinstalled at the center of travel position.
And it's hard for me to believe any qualified alignment tech worth his salt would not catch the difference in directional travel when doing the alignment.