Hi everyone. Look at this page at the parts in the circle showing a cross section of the sill.
http://www.mossmotors.com/Shop/ViewProducts.aspx?PlateIndexID=29038
Part # 13 is the inner sill, # 11 is the inner panel, #10 is the "rocker panel" and #12 underneath the inner sill is the "Castle Rail" which is a proper factory term. To change the rocker panel does require cutting off the dog leg which will most likely be bad as they are the very first place to rust out on a "B" because the dirt gets trapped between the rocker panel and the dog leg.
We have done dozens of these when we were in Canada but very few since we moved our shop to Ventura California. I have 2 B's in the shop right now. I had to do one car years ago that had nothing connecting the front to the back except the tunnel. No floors or braces and no sills whatsoever. One piece at a time and careful fitting with the doors hung.
Cheers, Dave
WELLWOOD AUTO RESTORATION, INC Ventura, California
wellwoodautorestoration.com