If you are heating up at highway speeds, check the deflector just behind the openings under the front bumper cover. Also, there should be a shield that runs from the radiator support to the anti_sway bar to help create a low pressure zone behind the radiator. These two elements help to direct air through the radiator at higher speeds.
The UD pullies should not be a concern if the rest of the cooling system is in good condition. Don't install them with a old, OEM radiator and expect not to overheat. From the design of the pump I would expect it to cavitate at high RPM, loosing most of its pumping power. I believe the MARCH pullies are the most aggressive and I have no issues with them on an 89 Auto driven hard, but I also had my radiator recored to a 3-row core, flushed the system well and refilled with distilled water.
Check your current radiator, if you have ANY mineral deposits in it you need to get rid of them, both engine and radiator. They will reduce cooling much more than pump volume will. Those deposits insulate twice, first to get the heat into the coolant in the block and again removing it in the radiator.
Aaron
PS: Mike - I agree that the UD set reduce too much, most of the gain is in the weight savings anyway.