I suspect the cost of buying the parts to build hopper cars from those shells will cost more than buying a bunch of Athearn blue box or Accurail kits. I think I would use the shells as an opportunity to learn about scratch building with brass. If you use brass square section tubing for the back bone of the frame, you could close off one end with a soldered piece of brass sheet, and fill the center section with lead shot followed by soldering a piece of brass plate to close off the other end. If you build your frame out of brass shapes, I think you might be amazed at how much weight you could hide in the frame so that you don't need any other weight for the model.