if you're looking for quarter mile performance (or 0-60) gears will be the best bang for the buck. won't do much in terms of RWHP but will make the car accelerate like it had 25% more torque if you're going from 3.27's to 4.10's.
underdrive pulleys on the 5.0L also used to be pretty effective in gaining some torque across the board (~10ft-lbs). i'm assuming the gains would be similar on the 4.6L.
-chris