Awwww, you're only saying that 'coz it's true..!
There has been some heated debate about Great Stuf not being particularly great for Skoolie application. Not fully curing, enhancing corrosion... You may want to search the site- there are recommended alternatives, can't think what they are, ATM.
Some good info on how to construct your subfloor here, as well. No point in re-inventing the wheel!
Unless you have a federal grant, in which case, roll with it!