3/8" probably not enough, but your current F350 has fully boxed frame members and the result is 24 times stiffer than older C channel frames. The result is less porpoising which is the culprit in cabover touching roof and clearance lights. Old guidance of 2"-3" probably not necessary with new frame design. I would suggest aiming for 1" and CAREFULLY testing the result, adding more clearance as necessary.
I'm concerned as I await my new 2019 F 350 and have to match my 2007 Lance 1181. Lance told me to consider horse stall mats to lift the camper.
Rick
