This is a bad mindset. You don’t want to start pre-production in a crunch.
Also.
Unless you’re the most experienced level designer on the team, you should either work together to come up with a workflow, or use one of the standard workflows. You’ve got it backwards, really. There’s also the fact that you’ve listed this as unpaid- but give a price for the game. Have you considered offering a royalty? Unpaid means that you’ll never pay for the work- royalty means that if the game makes money, they’ll get some of that money as payment. You may attract more people with royalty.
Anyway, just thought I’d say my piece and walk. Good luck!