I've always kept the Oxford Shell upright, fearful that the forces exerted at highway speeds would possibly break the coamiong boards. It fits my kayak saddles in an upright orientation. I would also strongly suggest tie ropes from the front of the Shell to you front bumper and another tie rope from the rear of the shell to the rear bumper.  This removes an enormous amount of stress from those two crossbars you'd mentioned.  Please be sure if you clamp the sculling oars to the rack that you cover them with some simple bag or cloth covering the blades so they don't vibrate at highway speeds. I also places a red flag on each end of the Shell, not for myself but others driving near you.  Always enjoy all the thumbs ups and admiring eyes of others seeing the dramatic and inspiring Oxford Shell.  Enjoy!

A long-time sculler (carbon racing boat) I recommend a proper carrier as made by Hudson. I leave the riggers on and it rides hull side up. pipe insulation around the shafts of the oars where they are tied to the rack help ease one's mind about chafe or chatter. And, because I once lost both carrier and boat when the *rack* came off the car, stiff as she is I use a wide strap over the hull tied to the bumper in front to prevent the wild oscillations that are frightening to look at and probably what contributed to that unfortunate excursion.