The FLM 3905 chassis which you have is the same length as the stock 3905 plastic chassis, so you shouldnt have much trouble adjusting the RC Solutions cage ( the 16.8v model is the one you want, for the new emaxx ) as it mounts to the front and rear bulks. There are more regular tube/ rod style cages about too though:
http://cgi.ebay.com/TRAXXAS-E-MAXX-E...item3369386ce7
they mount through the bumper-mounts so would be even easier to tweak for perfect fitment.