Go to any Autozone or Checker or whatever generic parts chain you have in your area and see if they'll let you pick through the boxes until you find one that suits your needs. I don't have the exact part number on me, but I'm pretty sure that the idler pulley on a '99-'04 V6 Mustang will be the right size outer diameter and bolt hole diameter to suit your needs, and it's ribbed. Take off your current pulley, walk in there, ask for that part for that application (don't tell them what you're really putting it on - they are simple creatures, 90% of them only know what's in their computer, so don't do anything that might confuse them), and then compare that with your current pulley.