Personally, it looks terrible, but, it is, I think, no different from the repairs already undertaken on less rusty cars. On a positive there doesn't seem to be anything requiring to be cut out above the top bend in the outrigger so you are only cutting out flat metal!
The process for the fronts is well documented (Ade's effort above seems to be most applauded but is no different to what Bob did, and I later copied

) in your case the metal insert will have to go further back and there will need to be some consideration if how to tackle the seatbelt mounts if they are also affected.
Basher did a repair to his rears, that at the time I confess I didn't understand but looking at the pictures on his photo bucket looks really good, which is also documented here. These pictures need to be downloaded onto the site (with Bashers permission) as it would be terrible to lose them and have to reinvent the wheel.
If the braces are also gone then these are easily replaced given that they are Sikaflexed to the floor pan but because of this I would start with repairing the length in the middle section (to include the front section if possible) and then the rears.
The other thought though is, because it is the length of the outrigger, it'll need to be treated as a seperate job for each corner doing the middle first will keep the rigidity of the floor where there may be some concern about ensuring there is no flexing when you remove the old rusty bits, maybe a length of wood jacked tight along the bottom of the cill?
I think that the start point, as with all good projects, is a plan. A step by step detail of the processes required into which you can incorporate any issues as you think it through - I'm sorry, that cliche about 'sucking eggs' comes to mind.
Good luck, daunting but not unachievable by any measure and needn't be too expensive in cash terms but probably more so in stress and time!
Pete
You know more than you think you know, just as you know less than you want to know. ... Oscar Wilde