[ChuckTin]

Where are you, is termites a problem?
Seems to me you're going about it correctly. Though pouring an in-place foundation is a PITA.
I'd support the wall from inside, pull out the 2 bys, form and pour a continuous footing with rebar tied in at the corners and j bolts for a PT sill plate, then cap the concrete with copper or galvenized flashing, new sill plate bolted down.
But that's me for our environment your MMV.

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[ChuckTin]

The ceiling rafters are parallel with the outside wall you're replacing?
Then you're going to put up your temp wall either directly under one rafter or between two. Either case you'll need to span two or even three rafters to be safe. Actually, probably over kill, but I'd build a 2x6 temp wall (am I chicken? Buck_buck, no) and make sure it was cross-braced as well. You might even, don't know what your roof (dead load) is, rent adjustable lally columns instead. May be hard to find tho.

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