Belt rubbing water pump

Anyone ever had problems on a 360 with the alternator belt rubbing against the water pump housing? Is it a tension thing, wrong alternator bracket, wrong belt size, or all of the above? I'll try to get a picture up later today - the corner of the belt's just being slowly chafed off by one of the ribs that runs along the water pump housing.

More details - the car has PS but no A/C - the alternator belt is only on the crank and alternator, while the P/S belt goes around the water pump too.