Now here's something interesting.
Jim, your D&D pipes, what is the diameter of these ? I would think that they are possible 4"
The stock Mufflers are some odd size, I did measure them (can't remember right at present) but think they were about 4¼" or something like that.
Your measurement of 16 3/16" would indicate a smaller diameter muffler compared to Norms TORS (I think) spaced at about 15"