Jack,
Thanks for taking time to answer. I had typed out a reply to your first part, but must have failed to hit "submit" after previewing it. I had stated that the only thing I might do differently was to perhaps extend the four rear seat mounting plates back to pick up the seat belt mounting points, per Juliano's spacing in an attempt to increase the continuity of the seat/belt mounting system. I will look closer at that, but you and I are on the same page relative to the separation aspect - take reasonable steps to keep the body attached, but if they do separate stay with the body, and don't ever become the cotter pin connecting the two via the seat belt. Since the upper shoulder harness point is a body mount, the choice is made for me.

Kitz,
Inertia is absolutely the key to this discussion, and if you have concern with your moment of inertia pulling through your mounting system then I would say that you need to beef up your under floor plates. I believe that it is impossible to prevent differential movement between body and frame in a severe crash, and I choose to tie my butt to the body, not to the frame.