Thanks for all the helpful advice everyone. The snow has melted, and I have begun excavation for my footers in each corner. My plan is to install 7 footers first, with the floor holding up the garage while I install these. Next, once the footers cure, I will break out the garage floor so the garage will be sitting on my footers.
For the footers, I'm using 12" sonotube forms and my question is this: Will a 12" diameter be enough surface area sitting on top of the virgin soil to hold up the garage and not sink once I break out the interior floor?