Welcome to the world of getting to know your Napa dealer. All you can do is take down your old belt and get one a little longer shorter then what you have, as long as you have the sleeve that the belt comes in it is not usually an issue to return so pick up a couple of different lengths to find out which one works best.
Do you have a manual tensioner, then wrap the belt. let it have some flex and then start the motor, let it run turn off and check.
2001 Camaro SS SOM M6, AES ls7 442, TFS 235 heads, 102/102 intake, GZ vacum pump, 13/4 to 17/8 kooks stepped headers. MW chassis fab 9" with aluminum center and wave trac diff.
Tuning is underway with the new intake so hoping for a better number.
550 rwhp 512 rwtrq.
11.23@130mph on a 1.70 sixty