you can also sand the belts down with sandpaper from the top while its running, making it thinner.
also if you dont have aluminum hex hubs for the wheels get some because plastic ones deform over time and scrubs the bearings, with aluminum ones they run truer and you can tighten the wheels down.
reading the "freeing up xray" thread, you might wnat to try cutting teeth from your front and rear diff ring or case off instead of your belts to prevent skipping, not sure if its safe to do it on the center pullies though.