We ordered our alternator overdrive pulley from www.rpmspeed.com. It’s part #540120 and it’s called a ‘98-’02 LS1 ASP Racing Alternator Overdrive Pulley. It’s 13/4" diameter and it’s recommended for use with LS1 cars with automatic transmissions and/or cars with high powered car stereos.  The pulley features 32% alternator overdrive which delivers increased charging capacity and it’s made of CNC-machined billet nitride; the price is $26.95 as of this writing. Please note that when using with stock crank pulley, a new accessory belt will be required: P/N-755K6. However, since our crank pulley (harmonic balancer) wasn’t stock, the belt we already had installed when we put the balancer pulley on worked just fine with the new alternator pulley. As far as tools go, you’ll need a 15mm wrench and socket, an impact driver and a 24mm socket for the alternator nut.
The stock alternator pulley is at the left, with the overdrive pulley at the right. You can see the huge difference in size.
The project car has a chrome alternator cover mounted on it, so the first task was to remove the two screws holding the cover on.