So , Hardchina , you didnt change just the inlet manifold , infact you probably changed the Throttle body , the Plenum and possibly the injectors as well , so which parts made what difference ? You honestly think any BMW engineer would have anything to do with that motor ? What this 4.6 ? 
The final version, used in the 
Range Rover, produced 222 hp (166 kW) at 4750 rpm and 300 ft·lbf (407 N·m) at 2600 rpm.
 
Now what manifold would I use ? A edelbrock or Offenhauser single plain 4 barrel with 1000cfm progressive throttle body on it ,Something Like this , but thats a 600CFM Quad throttle body for Straight LPG