2003 Cobra pulley/belt question

[Peters211]

Hey guys, i hava a 2003 cobra with a ported eaton and a 3.1 upper and i believe a 6# lower. i had an alternator go out, so ordered a new one and it didnt last more then 2 weeks. the fist hard pull to be exact, thats when i learned about the need for a bigger alt pulley. I ordered another alt, with a 3.7 pulley to slow it down the best i can! my question is what belt size should i be looking at for this set up? they say 2" bigger for the alt pulley, so would like to find out what others are running with the 3.1/6# combo, in the works of replacing the idler pullies aswell as they are still stock! Also, is this a good combo for a ported car? Thanks in Advance!