My 3G came with a 2.5" pulley, my underdrive pulley was the same size so the only reason I swapped them was because the underdrive one was aluminum. The stock 2G pulley is smaller, you could bolt it on as an overdrive pulley. I would see how it charges first, I doubt you will need to overdrive it.
I am having kind of the same problem myself...I have a 3G alt from PA performance and it charges much better than the old stocker but at idle sometimes,it hangs kinda low,like about 1/4 up sometimes a tad lower,other times like on the highway it's fine,charges just a about at 1/2 way up...
This website has a bunch of cool info concerning the 3G alt and mentions an ASP overdrive pulley #520004 3G alt info
Here is some more info on 3G installation in case you need it at all 3G install ...Be sure your wiring is up to the tasks of your 3G,you don't want any surprise engine bay fires....