I'd have a custom built street legal 917 coupe using one of the 16 cylinder engines that Porsche developed in 69 - 70 as an alternative to the turbo 12, but never used. I don't really care so much for the 800 HP, but the sound of an air-cooled flat 16 behind me could be quite amusing. Not to mention the possibility of laying rubber at 180 MPH.
http://upload.wikimedia.org/wikipedi...cyl_engine.JPG