there should be a slight rake empty to allow for loading, depends on the original GVW springs - ballpark on mine: the bottom of the front bumper is 15" [front springs may be sagged a bit] .... rear might have various bumpers or none, so with the fleet bed, it's ~28" to the
bottom of the tailgate, should be the same to the gate of a stepside .... or you could check the
resto-pak 
Bill
