We haul 2 Honda Ranchers in our 356QLB which is the same width as the 351. At first I was loading them side by side, but now I load them with one quad all the way forward and the other toward the rear. Seems to work better that way but still a pretty tight squeeze.
2012 Puma Unleashed 356QLB
2012 F350 Lariat Diesel
2-2011 Honda Rancher quads
1- Spoiled Rotten English Springer Spaniel