I'm building a brushless Losi of my own design. I'm using a Traxxas Summit 2 speed transmission.

Look how easy it is to work on my design. 4 bolts and the transmission is out! Clean strong and light weight. With no batteries it only weighs 10 pounds.
It's not done yet. I want to change the servos to the Hitec 7954 pieces that make 400oz. torque each! I also have to order the right output shaft to the transmission to get it running.

With the 9400 2s lipos on the sides it keeps the weight very low. it should handle very well on 4s!