It is simple math... your bed is 6' and the hitch is mounted in the middle leaving 3' from pivot to rear of cab. Your fiver is 8' wide. Meaning when you turn more than a certain angle that extra foot (4-3) will make for a bad day.
Three ways to solve the problem:
1. Slider hitch auto moves the trailer 14" back when turning
2. "Short bed safe" nose cones that are sloped and notched to create extra room
3. Extended pin box that moves the nose of the trailer far enough to allow it to hang further out over the bed rails when turning.
You can use any one or combination of them to avoid smacking the cab.