To complete the reinforcement frame rail boxing, the bottom plates are added. They are curved to fit the profile of the existing frame rail and are aligned using the bolt hole from the original cross member bar. For the plate to lay in flat and straight, the old cross member's built-in threaded nut had to be cut and ground flush with the surface since it was protruding above the surface.
Once the smoothing was done, I clamped the plate in making sure it was parallel and square with the other reinforcement plate. I tack welded and began to complete the perimeter welds with the MIG... crap! Out of MIG wire for now.