If you're talking about the cross sills, the front one gets bolted to the front panel, the rear one goes all the way to the rear. (That part was easy
)
One center cross sill on a 1/2 ton goes just behind the shock mounts toward the front of the arch in the frame. I could tell that by the wear marks in the frame when the blocks disintegrated and the cross sill settled onto the frame. That's about 24 inches behind the front mounting holes in the frame for the bed bolts. The other center cross sill goes toward the rear of the frame hump. The spacing will be dictated by the hole spacing in the bed strips.
Mar-K has assembly instructions available.
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