So would you be entertaining the idea of making a Spencer tang sight, since, as you said, you are only shooting 100 yards, why not stick to the original manufacturer's pattern tang sight? One thing that I have noticed though with regards to the rifles fitted with the tang sights is that often the rear sight is removed because it gets in the way of using the tang sight. If you are shooting only at 100 yards, you may not really have need of a tang sight?
Is this Spencer being designed as a full-size replica or reduced scale? And what CAD software are you using for the design? Autodesk Inventor is a nice CAD-CAM software package, though I am currently doing most of my 3D modeling in Bentley MicroStation, I can export to Autocad and then import it into Inventor for the final production model.