Brace yourself.. maybe?

[1000wannabe]

Hi Everyone,
I'm facing some rust repairs that were uncovered during carpet removal, primarily the carpet glued to the inner sill and the wheel well.
It turns out that there was a hole in the driver's front wheel well that was left for a long time and subsequent mud etc got into both the inner sill and outer under sill/boxing plate area.

I need to replace the inner under sill almost back to the cross member as its crusty and about the first 5 inches of the inner sill from the wheel well back( where the carpet was).
Removing the outer kick plate, I've been able to ascertain that the boxing plate and outer under sill are all fine, they were just caked in dry mud. The car has spent the last 10 years off the road and I believe that saved it.

I already have a new inner under sill panel with extended floor edge and a complete inner sill panel with seat belt mount arriving next week.
I've already offered up the inner under sill panel and I can retain almost all of the original floor, its only a bit iffy where the inner sill sandwiched to the floor. (the half inch horizontal edge of the inner sill)
My plan is/was to cut out the rotted inner under sill section coming into the floor towards the seat mount by about an inch and then back to the cross member and use the full length of the new panel, weld this in place and weld it to the floor. Then look at cutting out the front section of the inner sill, likely no further rearward than the beginning of the door aperture ( A pillar area). I will only make use of the very front section of the repair panel as I can see through the boxing panel that the rest of the inner sill is fine, welding it to the wheel well arch that is in the footwell, floor edge (plug weld) through to the now new inner under sill, sandwiching it all back together as per original.

As I don't intend removing any more than one piece of the overall sill structure at any one time and the boxing plate, outer under sill and most of the inner sill will still be intact and connected to both A & B pillars etc, is it necessary to still brace across the door aperture when cutting out the front section of the inner sill (where the carpet was)?

Its a fully assembled 4 door 56 series 2 and I don't really want to have to pull it all apart if necessary. Obviously carpet and seat are out...

Cheers
Chris