Will my 2003 A8 4.2 Quattro gearbox fit on a 3.0L 2006 A8 ?

 

Probably not. The bolt patter is likely different and since the two ECU's (the engine and the trans TCU "talk" to each other, some programming may be required to make it work. You must take the TCU (Trans Control Unit) with the transmission it is programmed for with it. It can get a bit complicated even if the mechanicals work out. Gone are the days of "if it bolts up, it'll work"...