yes you can still mount a 5th wheel hitch in the bed. I wouldn't use any goose neck adaptors they put a lot of stress on the 5th wheel frame. depending on the truck there may be a couple options, the first being a traditional 5th wheel hitch which is mounted to the frame via holes through the bed. the second is a factory puck system where they would lift the bed and mount a sub frame then drill the access for the puck mounts in the bed. I did this with my ford and it is the best set up as when your hitch is out your bed is fully functional and you don't have the rails left in there to work around.