For the amount of re-engineering necessary to do the V12 it might just be easier (dare I say economic?) to buy a Brabus 6.0 or 6.5 engine (both M119 based V8's) for the car and not have to redo everything. I know the 6.0 V8 is good for at least 420HP and bags of torque. Don't have 6.5 specs offhand but you could bet it is similarly awesome.
Just a thought...Lee
PS then you could rightfully put one of those little black "B" badges by the 400SE on the decklid.