Brent:
The metal pieces are the bearing plates for the aftermarket cross sills. On the factory cross sills they are spot welded on the bottom of the cross sills in the locations where they contact the frame rails. They are sold loose on the aftermarket cross sills because on the factory cross sills each sill has a specific location thus the bearing plates have a specific location. I bought a set of cross sills from Chevy Duty for my '66, C-10 and it came with these bearing plates. The bearing plates distribute the weight of the bed and any loading equally to both bottom flanges of the cross sills and keeps them from collapsing or compressing. They install between the cross sill and the frame rail with the bed mounting bolt going through the bed wood, cross sill, bearing plate and frame rail.
Good luck with identifying the rubber gasket. I don't think it goes on a '64-'66, however I don't know about other years.
