Joseph, good point about the different speeds. I've seen many trucks that specified different speeds on the data plate. I've even seen these discrepancies on M809 series dataplates. Some say MAX SPEED 50 MPH, others 52 MPH and yet others 54 MPH.
With regard to the OD tranny vs. the direct drive tranny, if you are driving on the highway...empty...the OD tranny is better because you achieve higher road speeds. If you are going to be hauling heavy loads, I much more prefer the direct drive transmission. Joe Young once had an M62A2 with an overdrive tranny. You could not shift her into fifth without a slight downhill grade and a tail wind! However, I had an M543A2 with the direct drive transmission; that truck would easily pull up to 45 MPH and maintain those speeds on reasonable highway grades. The negative...45 MPH is your max permissible road speed. Ask me how I know? I blew the head gasket by trying to maintain 47 MPH on the highway because I was not used to the direct drive transmission. So, if you want higher road speeds, go with the OD tranny. If you want more power, go with the direct drive tranny. I prefer the latter.