i can say this mod can be a pain in the ass. so i will tell you what you need to do exactly with the part you get from advanced, auto zone, etc.
First if you have a l99 or ls1 go to advance ask for part number #34209 its a doorman/ motormite a/c bypass pulley and bracket fully assembled.
second remove old a/c compressor 3 bolts in front cant remember size but belive 1/2 or 9/16
you will have 2 10mm bolt on the rear of a/c compressor have fun with these but doable.
next remove a/c lines, accumulator cut bracket for accumulator to hold just power steering res.
after all lines and a/c components are removed then get your bracket and take a 3/8in drill bit and drill out threads on bracket. (very important or will not bolt up properly. second you need fender washers 3/8 x 1/16
take your 3 bolt and put in bracket make sure they slide smooth and place 2 washers on all bolts behind the bracket this will give you the 1/8 in difference between the bracket and original design or it will squeal and blow your bearings.
after it is on and belt is secure start it up and go. thought i would throw this info out there since some people dont know this i didnt and went through 3 pulleys lol.