between the 3905 and 3906 can you swap the transmissions? Is there any slight modification or is it a totally different beast? Let me know, im pondering swapping my worn out 3906 chassis with a 3905.

They are totally different

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!!