what year bed wood strips?

[jdl71]

What is the difference in the 60-62 and 63-66 strips? I know the frames are wider on earlier trucks so the cross sills would be different but how does that affect the strips? Is the front to back spacing of the cross sills also different on the earlier trucks? The wood planks are the same width so I can't really see what the difference would be. I have a 66 truck with at 60 bed and cross sills and am getting ready to replace the wood.