The frame holes where the steering gear is mounted are slightly oversize. Center the wheels, then loosen the steering gear and move it as required to center the steering wheel. If you cannot center it, check the frame for bends/damage. The FSM gives the checking dimensions in the "Frame" chapter. Use a plumb bob to project the checking points onto the floor, and make the measurements there. You can project the points onto a sheet of paper or cardboard, or just put some pieces of masking tape on the floor. iirc the steering columns on trucks do not use a master spline, so the steering wheel can be relocated relative to the column.

Ken