I think you'd need to mount the rack on the axle for the geometry to work without massive bump steer. Could be quite difficult to arrange a steering column setup that can deal with the total 5" or so of axle/rack movement.
I have the 70s/80s Truck Saginaw setup on my AD. We are going to fit the same setup to my brother's 36 low cab truck later this year, doesn't look to be too difficult as there's room on the front of the chassis for the steering box -would have thought your GMC would be similar.
Stuart.
