This is a rough idea of the back of the desk. The wall you see is 12 inches from where the actual wall will mount. so the motherboards will be mounted 6 inches out.
Heres an ugly sketch of what that back wall would be. the hose idea was because i want everything symmetrical. The power supply is and HD's will mount behind the motherboards. the radiators i haven't decided where yet. probably below the motherboards.