I would recomend an 8XL for running on 4s 8000 and MM.
As for layout, sounds like you want something like BrianG's Hyper 8. My 8ight has the motor on one side, battery on the other.
My 777 has the motor over the center diff like that, but I did custom driveshafts and moved the diff forward so the batteries could tuck in behind the center diff. Brian' sis similar but leaves thd diff centered in the car and the batteries are a little farther out from the center.
I really like the 777 layout but it does require custom shafts... more work to do. Some pics of my cars....
http://www.rc-monster.com/forum/showthread.php?t=5572