1955 -59 (2nd Series) Chevrolet / GMC Stepside Truck Cross Sill
This cross sill is mounted across the frame under the wood for support. The bed strips are bolted to the cross sills giving the bed wood strength and rigidity and providing the surface to mount the bed to the truck frame.
Each sill has pre-punched holes in 14 gauge steel with the original bolt pattern for your year and model.
1955-59 2nd series GM Stepside truck:
2 required for short bed models⁕
3 required for 89″ long bed models
4 required for 97″ long bed models
⁕ Some 1955 2nd Series to 59 short bed stepside beds originally had a 3rd Cross Sill that was different from the others. The outer end of this sill is formed upward to give clearance under the sill and outward from the frame. Your truck suspension may or may not require this sill.