One goes just before the frame kick up. It only bolts to the bed wood. It clears the frame by about a half inch and comes in contact with the frame only when a heavy load is put on it. Its location can usually be determined by locating a worn place on each frame rail just in front of the frame kick up, this is where the cross sill came in contact with the frame during a heavy load.
The other cross sill installs just behind the frame kick up using a 2 inch wooden block. Its mounting hole should be the first hole just behind the frame kick up.
hope this helps
