Originally Posted by Pete_vB
Going to anything other than a BMW motor would be substantially harder due to the incompatibility of the electronics. My personal choice would probably be a tuned S65, not supercharged (to keep the weight down), maybe stroker or built with cams and a 9k redline. Any heavier motor, including the S85, would adversely effect handling.
But as long as you're dreaming, you might as well use the S70/2 V12 that BMW developed for the McLaren F1. 680+ hp @ 7800 rpm in LM trim.
The McLaren F1 engine is definitely the stuff of dreams, aurally nothing comes close to that level, except the Pagani Zonda. Which company offers the S65 at the level of tune, or can modify one to the specification you described. Also what are the gains over stock.
I read that the LS motor (with supercharger) weighed 197kg, vs the S65's 202kg.