I remember flatbeds arriving mounted on chassis at dealers. Just about everything else arrived as a cab and chassis. One of the old manufacturers is still around. Ace Trailers in San Antonio. I haven't been there in 25 years. I don't know if they can still make the corrugated side rails or not. I don't think there is enough steel in a bed to make the weight difference for aluminum worthwhile. Aluminum cross members are still common in van trailers and could be either bolted or Hucked to the side rails and insulated with mylar tape.
