lightweight pulleys dont GIVE any power, they reduce drag, you wont gain any power but you may find the car will pick up a little quicker.
im not sure if its the same with the 350 as im used to 200sx's, but they have a built in damper in the crank pulley,(cant remember the technical name, resonance dampner???) but when people were swapping out for lightweight pulleys they were finding the main bearings went soon after, but not sure if its the same with the 350,
let us know how you get on, and yeah under drive is exactly what people have already said, you may find you need to find differant belts tho as the pulleys are a differant size