IIRC, the frame mounts for the cab/box are higher on the HD frame (the old 2" body lift GM advertised), and the rear section of the frame under the box is taller on the HD frame. Those are the two differences I saw over the years.
The front of the seam under the cab, yes. The rear portion from the seam necks down under the bed on the 2500. The HD frame stays the same height all the way to the bumper. That is what Ive seen in the past, but as usual....double check first.