Many people on stangnet have told me ASP are the best for 5.0, they are 74 bucks, you can find the cheaper if you look. Someone on the forums dynoed 9 rwhp with just pulleys, but the car was stock.
EE sells them (one of our sponsors) im sure many of the other sponsors do too... and if you send them a pm about somethin youre wanting, usually they can give you better deals than the list price
The asp underdrive pulley is a very hot item particularly for the v6 RPM-mustangs has a backorder from over a month ago. I don't think any one has them right now (v6).
my underdrive pulleys aren't killing the alternator.. but it can't handle having the headlights on, radio and turn signals at idle without having the car dip into battery reserve.. just a FYI