if it's a street or drag car, leave it stock, or even heavier than a stock one would be nice. The lighter ones are good for road racing where the engine is constantly accelerating/decelerating (the ligher mass makes it "easier" for the engine to do so). Actually, in drag racing, a heavier flywheel is better. When you dump the clutch at for instance 5000 rpm, the engine wont drop as many rpm due to inertia as it would if the flywheel was 20 pounds lighter. This will make the car leave the line better, traction permitting.