The 2007.5 models marked a change in body styles, and a 2 inch rise in bed heights. It doesn't make the trucks look better, only increases the cubic capacity of the truck bed, but screws anyone with an "in the bed hitch". This is why you have less clearance with your new truck. You can probably get by with five inches, just be careful off road, like when backing into a parking spot. The usual solutions have already been posted, just be careful that you don't get the nose of your trailer too high while trying to gain bed rail clearance, as this will create a whole set of new problems related to weight distribution and overhead clearances.