Just to be contrarian, I think you'll be fine with a smaller flash back stage. It'll be pretty dark back there, and you'll want turn it down far enough to pick up some of the ambient light. You'll also be pretty close to your subject, so too much flash is going to blow things out. You definitely want to use a wide lens, a 18mm lens at a minium, and turn up the ISO to 400 or 800.
I always shoot RAW, unless the client asks for JPG's. I would go early and get a custom WB reading, as who knows what kind of lighting they'll be using. If you're going to be using flash for the runway, you will need a big one.
Good luck,
David Buzzard