They are different. You'll need the new trans as well as the new chassis and some of the 3906's drive train. Why not just buy a FLM Extended Emaxx chassis to replace the old 3905? It would prob be chaeaper than teh new chassis and everything else you'd need to make it work.

Thats what I was going to do and liked the FLM chassis so much I just built a completely new one with the FLM chassis - went brushless at the same time too!!