Unless I come up against a major problem that I can't resolve, I am going with one of the Mercedes 24 valve V8's. In order of preference they are 55 AMG, 500, C43. 430.

320 215 HP 229 foot Lbs Torque (Stock)

430 275 HP 295 " "
C43 302 HP 302 " "
500 302 HP 339 " "
C55 342 HP 376 " "

An ECU re-flash will add 20-25 HP to any of these engines (provided I can get 93 octane). Cams can add another 25 HP.

The weight difference from the V6 is 130 lbs.