It takes a lot of muscle to move a Bentley sedan, which can tip the scales at three tons. The company's own V-8 has answered that call for 61 years. But no more. Bentley technicians in Crewe, England, built by hand the last of the whisper-quiet, silky-smooth 6.75-liter V-8s last week. It was installed in the final Mulsanne 6.75 Edition by Mulliner sedan.