With that shark nose, depending how much time and money you want to spend, you might want to try to make your way toward something like this:
Or this (taller, of course):
It is generally said around here that you want a low stagnation point, something the semi truck doesn't have. Do this by angling the front surface on your vehicle backward to let air flow over and around the body instead of over it.
As for the air dam, I made mine slightly curved and I have the same front bumper as your van. I don't know if it really matters that much, but you may as well put a sublte curve in it if you can.
As was said earlier, you might consider a box cavity on the rear. We haven't had many EM'ers try them, and I never got a good chance to test mine on the Probe. Will you be the first? Otherwise, boat tail!