Got it from alternatorparts.com and their help desk is helpless.

I'm having a problem in that either it sits lower than the stock alt. or the pulley is smaller. Either way, my serpentine belt is too long (I need about a 62.5 to a 63 inch belt). Problem is, it seems neither Gates, Dayton or Goodyear sell belts between 61 and 64 inches. So I put on an underdrive crank pulley. Now I need a belt about 57.5 to 58 inches. Same problem, no belts between 56 and 59.

So, has anyone installed one of these and if so, what did you need to do?

 

If you still have your stock alternator, try that pulley on your new alternator. Or try the stock crank pully with an alternator overdrive pulley.