Oh... this just in from Lance:
"Yeah. It should be an exact match where it meets the neck tube and then expand at the same taper to match the body tube perfectly, as if the neck had never been cut off and that 1" section replaced with a cylinder. The cylinder results in normally a 1.5mm diameter bore error where the tenon meets the body."
I had asked: ...still not sure about upgrading the tenon? Do you mean it should follow the shape of the body tube bore, so it is wider at the body opening and narrower at the neck side?
I think I have it now. He's saying that as the original tenon is cylindrical, it's narrower than the body receiver whereas it should taper outwards into the body socket, matching the body tube, and also the neck side, diameter.