EDIT: Sorry I didn't see that it was for a KB. The 100852 works on the stock blower with a 10% overdrive and the 2.9L with a 10% overdrive. Not sure about the KB though.
Is the 852 belt too short or too long? If its a little long them just replace the lower left stock idler with a Metco MIP-90 ilder. It will take up the slack. If its too short..... You're gonna need a new belt.