Most everyone is changing lower pullies because the upper is a stamped piece that is easy to screw up when removing. You can buy a 2 to 6 lb. lower pulley from one of the tuners below:
www.johnnylightningperf.com
www.powersurgeperformance.net
www.teamjdm.com
They all have quality stuff but I think you will find Johnny to be the most helpful.
You may want to check out
www.f150online.com or
www.svtperformance.com as many on these boards can give you first hand info on install and experiences.
HIH