I too wonder what all was done to the Thunderbird concept to get the 390 hp/ft.lbs of torque rating. It appears from the writing below that the headers were the only thing added to the engine. This leads me to believe that the stock 3.9L internals should be pretty strong!
FORD THUNDERBIRD SC CONCEPT:
With 390 horsepower on tap, the Ford Thunderbird 3.9-liter supercharged V-8 engine transforms the vehicle's character from a grand touring car into a performance-minded sports roadster. The supercharger setup offers peak horsepower at 6,000 rpm and 390 foot pounds of peak torque at 3,500 rpm, giving the Thunderbird concept car impressive power and acceleration capabilities.
Ford's powertrain engineers began the supercharged concept's transformation with a stock 3.9-liter DOHC modular V-8 engine from the standard Thunderbird. Then, all-new aluminum headers were fitted along with a Roots type supercharger to provide the necessary thrust.
"Achieving 390 horsepower from a relatively modest 3.9 liters is a testament to this engine's efficient use of technology," says Dave Szczupak, Ford Motor Company vice president, Powertrain Operations. "The raw power of this most powerful Thunderbird ever is intoxicating to sample, but our designers have made the Supercharged Ford Thunderbird just as exciting visually as it is to drive."