Mar-K has a set of instructions for sale on their site. They are excellent. Briefly the cross sills are located 2-1/2 inches, 26-11/16 inches and 53-11/16 inches from the front of the bed. There is a bed to frame bolt at the first cross sill. The second cross sill (the one just behind the shock) does not have a bed to frame bolt. The third cross sill has a bed to frame bolt. Finally the rear bed to frame bolt passes through the brackets attached to the rear cross sill.