So from another thread, here is a quote from captainfab:
"Yes the plates on the bottom of the cross sills basically sit right on the frame and the mounts that are on the frame. There are rubber pads that you can buy that go between the cross sills and the frame/mounts. I have never seen a factory truck with them, but others have. Some of the plates on the cross sills will overhang the frame or the mount some, but as long as the holes line up, you're good. Also the mounts that are spot welded to the rear cross sill are supposed to sit on top of the frame. Take a close look at the pics that earl84 and 60-66 posted above."
Here is a link to that thread:
http://67-72chevytrucks.com/vboard/s...d.php?t=628105
These are the pads, although I don't think our trucks came stock with them and certainly much smaller than the ones you have
https://www.tuckersparts.com/bed-mount-pads-60-66/