Do you have a Mustang that you use only for racing? Then why don't you get rid of the air pump? They're not needed on racing vehicles, and you will reduce the front-end weight, too. These Ford Racing smog pump eliminator bracket idlers are direct bolt-on replacements for stock air pumps, and they use the stock serpentine belt.
Ford Racing 1979-93 Mustang Off-Road Idler Bracket
COMP. A/P IDLER
Engine: Cooling