Squealing fan belt

My 72 Dart with 318 a/c and PS has always had a problem with the fan belt squealing on a hard acceleration. The belt is from Napa and is a 15375, I believe. I know the tension is correct. the PS box is new as is the fan and fan clutch. The engine is not leaking anything on the belts or pulleys either. I am at a loss.

Scott