The 6' 9" bed will definitely help on the clearance issue. However, to be sure, I would back the truck under the 5er at an angle of approx 45 degrees, watching carefully, and position the hitch pin directly over the center of the rear axle. Then measure the distance between the corner of the front cap and the truck cab. If that distance is less than 1 foot, you will probably need a slider. If more, then you probably will not have to have the slider. JMHO, without actually seeing the unit. Again, the purpose of the slider is to move the pin distance toward the rear when you are maneuvering with tight turns in a CG, so the 5er will not hit the truck cab. Then when you are on the road, the slider puts the pin back over the axle for traveling. Hope this helps.
2003 Ford F-350 V-10 Crew Cab 4WD Long Bed
2004 Jayco Designer Medallion 29 RLTS 5th wheel
Bill, Gayle, and Mindi (dachshund)
Retired at last !
Nothing is foolproof to a sufficiently talented fool.