Throw a jack under the front end of the truck, and use a large level, placed under the cab, under the door like wherea step would mount. That's the most level/straight part of the truck. Get that leveled off...then put it on the bed. See how far off centered it is, take measurements from the front corner and rear corner. If it's a 1/4" higher in the front..you know how much needs to be fixed. Be much easier that way. That's how we leveled my '02..by using a level on the top of the bed, right behind the cab. Bastard was just barely like 1/32 off center, to the front with a leveling kit and f350 blocks (i used a 99 dually 4x4 vin number).