Do you have the compressor for the Red Dot aircon installed?
So just putting this information out there in case anyone else needs it. My truck is an M1123 with the 6.5 engine and serpentine belt setup. It has the Sanden compressor for the RedDot aircon in it. That part is probably important, as the belt length does not match what the manual says it...
www.steelsoldiers.com
You can also take a picture of your belt/engine showing the top passenger side where the red dot compressor would go, and of the power steering pump and reservoir.
Pre 196900 engines use the power steering pump with the built in reservoir.
Post 196900 engines use a remote reservoir for the power steering pump. There is also an extra idler pulley between the generator and PS pump.
Even if you have the lower serial number, sometimes the newer engine setup gets put in anyway, so that's why you want to make sure which power steering setup you have.
So if you have the pre 196900 setup without the Red Dot compressor, you can use
https://www.napaonline.com/en/p/NBH25080981?impressionRank=1
If you do have the Red Dot compressor, then you'll want to use
https://www.rockauto.com/en/moreinfo.php?pk=102328&jsn=3