Best way to support walls of existing wooden outbuilding
This is one of those &quot;best&quot; questions that's highly dependent on site conditions and your capabilities and prerogative. It also depends on how much footing you actually want (or need, if frost heave is a concern). I see two options:

Do as you suggest, first pouring footings and then setting block under the wall plates. This would require someone with some good block-laying skills. That sort of bottom-up work isn't easy. You'll need to fill in around your temporary legs, then remove those and close up the gaps.

Pour an integrated footing with the slab. Once you backfill and compact for the floor, shape adequate footings forms into the soil below the walls. When you pour, agitate the concrete well so that it flows under and against the wall plates. Ideally you'll do some of this from the outside to eliminate air cavities.