52:
There is a "front" cross sill that bolts to the angle strips and to the front bed panel. It has a row of holes on its front facing surface to bolt to the front bed panel. The next one toward the rear is about 26 7/8" back from the front bed panel and the third one back is about 53 7/16" back from the front bed panel. There should be an angle strip hole at each of those locations and the edge board is sandwiched between the cross sill and the angle strip. Use 5/16 carriage bolts at the angle strip and 1/4" hex bolts at the front bed panel. Only the cross sill at the front and the one at 53 7/16" get a bed-to-frame bolt, the middle one just rests on the frame but is not bolted to the frame. Hope this helps.

Mar-Ktech