I don't think there's much you can do here. I tried to do the same exact thing 2 weeks ago with Dollar, but wound up with the mini van. For some reason, those 8 - 15 passenger vans always go for a premium and rarely get discounted. I was even checking Orbitz on a daily basis for a couple of months and never saw those vans get discounted. While asking at the rental desk, I was offerred the "Grand Voyager" which is extended and has extra cargo space and seats 7. I think it was an extra $10 per day. That's one idea I can come up with that may work out for you.
One other idea that I have heard people do (albeit more cost) is to have the other 6 passengers take Mears or a van service to WDW while the driver takes all the luggage in the mini-van to the resort, and reverse that plan on the way back to the airport. Tiffany has a luxury van that will cost $90.00 round trip, and make a grocery stop on the way. $90.00 is a lot cheaper than the difference between vans. That's probably what I would do.