i bought the 2 pulley set, but i heard of charging problems with the alt pulley.

So my question is what is the part number for a new belt if i just do the crank pulley and leave the stock alt pulley on???

 

Depends on the exact underdrive ratio of the pulley, which is determined by the diameter of the pulley. Without that info, no one can tell you what size belt you need. Best thing to do is put the pulley on, and see how well the stock belt fits. That will tell you how much shorter the new belt will have to be. Or wrap a string around the path of the serpentine belt, and measure the length.

If you bought a 2-pulley set, was the alternator pulley an underdrive pulley, or an overdrive pulley? March, for example makes both. The overdrive pulley speeds the alternator back up to support vehicles with high engine loads. Of coarse speeding the alternator back up gives away some of the tiny HP savings you actually get with underdrive pullies on an LT1..... the whole thing is really not worth it.