Between the bed rail and the fiver, we have about 7 inches and haven't had a problem. Don't know about the tailgate spacing. Our truck bed is the 6 1/2 foot. No problems and this is our second short bed using the Superglide. I would be cautious if maneuvering caused a severe sideways tilt of the truck or trailer. Always take your time and get out and look first.
Dave and Ivy
2020 North Point 315RLTS
Ram 3500/Aisin CC, SB, SRW, Superglide
Firestone Ride-Rite Air bags, TST507 TPMS