I believe the bed wood should only have holes drilled along the outside two planks for the length. Two other boards will have holes for the anchor bolts to the frame. It was this way for my one ton. Weird that your cross sills don't have holes in the top for the bed strips to bolt to.
Perhaps wait to drill holes unless you're sure of everything. We'd hate for you to wreck your boards! Someone who has more 1/2ton experience will chime in I'm sure!
bought the sills a while ago and didn't know better
a lesson learned
( had no old parts to compare to )
