I have a twin screw blower on a 2006 GT mustang and love it. 7.5 psi, and the hp went from 300 to 480, and the torque is right there on the bottom, starting at 2,000 rpm. The car ran 115 mph in the 1/4. Just think what a car that is 500 lbs lighter with a much bigger motor would do. The twin screw eats a little power over the turbo, but the response is instant, and the torque down low is much superior to a Procharger. I would go for a 500 cube motor with the proper sized Whipple unit.
As an example of how well they work, the STOCK class record for a new Cobra Mustang racer is 8.05 in the 1/4!
That is with a 4.0 liter Whipple on a 302 four valve motor.