an idea that I had was get the 540 motor mount, and get a mid to low level 540 motor so you don't kill the drivetrain
then take that extra space on one side, and stack 2 LiPo's and run them in paralell...
then put the ESC behind the motor, and the RX on top of the servo(s)

since the space is long, you could get the longer, flatter packs made for oh, say the RC18. That would be good for me, since I am going to be running my M18 on LiPo too. Won't need to buy 2 seperate kinds of batteries.