Sounds like your sills will need the full monty. Not technically difficult to fix (probably easier than the mini) but a fair amount of work. Remove rotten inside panel and replace with angled panel (135* I think) and seam weld to floor panels and inside panniers/pontoons. You will probably have to replace flat section underneath as well. Also check section/corner hidden by the fuel tank removable panel.
The box sections for front subframe mounting are "factory" and used to bolt the subby to the body via the recessed holes in the front of the floors. Another great rust trap. Replacements are probably NLA so you may have to replicate using heavy guage steel, if yours are beyond repair.
Look on the bright side - your welding skills will improve immensley after fixing all this, as did mine
