@Cain: rear hexes are easy. All of the pictured hexes fit right on no additional mods or spacers needed. I've shown the proline 12mm hex, the JConcepts (same as proline), avid and lastly the tresrey.
Using the AVID set is nice as you get two different widths of hex and thus allowing you to change the rear track a bit.
http://www.avidrc.com/product/5/acce...cessories.html
This is also the bonus with the Kyosho 12mm front axle upgrade. This allows you to adjust the front track a bit.