I need assistance from those that may have had the same problem. I have a 5er that used to be towed with a 2005 Dodge Quad Long bed. Recently I snagged a "used" 2014 Chevy 3500HD dually.(only 7000 mil...
You have the same issue I had with mine. I ended up cutting the corner of the pin box to get the clearance needed to not hit the bed rail. I had less than 1/2" between the pin box and bed rail when turning before I trimmed the corner.
It wasn't hard to do, took about an hour including painting the raw edges.