In the past we have used a hub (found on
www.firstcadlibrary.com under mechanical) with a sprocket attached to it. You will probably need wheel wedges also (found on
www.firstcadlibrary.com under manufactured). Last year we attached a gear to an aluminum adaptor to the hub to drive our tri-wheels. You may want to machine the hub's yourself out of Delron because we found the "MMH" version to be too weak. They broke apart quite eaily because they are molded.
If you need some more 6" wheels for your idea let me know. We have a bunch here. Good luck with your 6 wheel drive, I can't wait to see it run. If you need any help let me know.