The curse to carrying out repairs is encountering previous "MOT" repairs (where plates are welded over rusted holes).
Attached are photos that I hope are helpful, Ken.

Firstly, I trial fit the outer sill (using self tappers along the top edge. This is a TDC supplied sill which is a good fit.
Photo shows back of the OS sill.


These photos are views of the sill structure with the outer cut and pulled down to reveal the inner structure.

To form a box section for reinforcement at the jacking point a plate is welded between the inner and centre sills (at right angles). It is just visible (highlighted). The lower section (where the jack meets the inner sill) is also reinforced.
Use decent sheet steel for patches (1.2mm thick or 1.6mm thick (my choice)) and don't cut out too much all at once.
Judging from your thread so far, these repairs shouldn't present too much difficulty, but they are time consuming.
Hope this is helpful, Ken.