The '04-06 E53 3.0i was available with a 6-speed manual and X-Drive. I have no idea if it bolts up, or how much torque it can handle.

The 3.0L auto tranny is matched to the engine output. If you really want to try thr conversion somebody like "Metric Mechanic" who market specialized drive components and engines might be the best to contact.

Talk to these guys: http://www.bavarianperformancegroup.com/

I talked to them about changing the final drive on my 5-speed E53. They seem to really know their stuff, gear-wise.