As finally stated, the issue is with length of the pin box. Most fivers today have extended pin boxes and many of those have a length that is greater than 1/2 the truck bed width, so contact is definitely possible. Also, as indicated in some cases there is excess metal that can be trimmed for more clearance. Some designs are tapered in the back and some are square and the tapered ones will require less vertical clearance for the back of the pin box to swing over the top of the truck bed rather than hitting it.
I use a utility bed, that like the Rambox is only 50 inches wide, so I have lived with this issue for many years. But in my case I have almost 10 inches of vertical clearance and the stock pin box swings over the top of the utility bed. This does not work with the Rambox as it is too tall to achieve that much vertical clearance.
The one solution that no one has mentioned that will work with either 50 inch or wider truck beds is the Sidewinder pin box. Since the pivot point is moved back under the fiver, there is very little side swing. Then there is the original fiver design solution, an 8 ft truck bed and a non-extended pin box. Finally there is the option to remove a stock truck bed and replace it with a flat bed or hauler bed. So, there are plenty of solutions to this potential problem.
