Cooling fans and squealing belts

[xaircav]

As you know I have a '62 283 in my '53 five window. I recently did an alternator conversion. If the cooling fan is installed and I blip the throttle the fan belt squeals. I have tried several different belts, a six blade fan and now a four blade 15" low drag fan. Removing the fan removes the squeal so the extra weight of the rotating mass must cause the belt to slip. I'll need to put the fan back on when the weather warms up but I'm sick of the piglet sound. Doesn't squeal at idle or cruise.