I use the Napa belt number 060623. The demensions are 21mm by 1595mm. My experience with this belt is that it is super tight when new. We bought a O'Reillys brand 1595mm and it would not fit. To install this belt you better have a helper.
HTH
When i got mine acouple years back it was the last one in GM stock Still have it all nice and new and I am sitting on it as well.
I'm sure they are hosing you on stock blets bad enough(if any left) to warrant trying aftermarket. By the time I need another one, it will be all that is left.