Just went through a similar problem with my hot rod; it has a small diameter pulley on the alternator to increase the speed and, therefore, the output. The 3/8" belt didn't sit all the way down into the pulley groove, about .050-.060" of the belt sat above the groove. I replaced the pulley with one that was a little larger in diameter and a has a deeper groove.

I theorize that the increased current draw from the A/C increases the load on the alternator and causes the belt squeal.