I don't think there's a master spline for the steering wheel on those trucks, so just because the steering wheel is straight doesn't make the box centered.

Most steering boxes are designed to have a little less lash in the center of travel vs the ends that's why they should be checked and adjusted on center.

I have an 80 Ramcharger I'm working on, someone adjusted the box wrong and you can feel it bind up as it goes past center