sandblaster or wire brush + rust neutralizer?

The floor is part of the structure. So I would do that prior to or at the same time as body work. Floor boards are not hard to replace, you just need the right tools. Sometimes you don't need to replace the whole panel, just scab in patches to replace the rusted metal. If you choose to do it all, then stripping the car to the unibody then having it soda blasted becomes a more viable option. If you have a collage or high school with a n auto shop class, you might get the work gone for cheap. My local community collage has a class specifically for working on your project car and have access to tools and know how, you supply the parts, labor and act under tutelage.