Serpentine Belt Replacement - Duramax 6.6 Diesel

[Ol'Jim]

The tensioner pulley is shot, so I just went ahead and replaced the entire tensioner assembly, and also bought a new belt. I realize this is the "worst case" as far as making a belt installation difficult, because the belt has not stretched at all, so it is on the short side, and the tensioner has a much stronger spring in it than the 9 year old original one.
I absolutely am unable to get the belt over the alternator pulley, with the tensioner at about the max limit of it's throw. Yes, I have a belt tensioner tool, but it seems like the belt is an inch too short. When I bought it, I brought the old one to the parts counter, removed the new one from it's sleeve, and did a basic length comparison right there. Not a precision check, just looped both around the bottom of my foot, and extended them upward in my hand. They appeared to be identical in length. I also used a cross-reference to make sure the replacement was a direct cross to the OEM / GM Part Number on the original belt.
Wondering if this is a case of 2 aftermarket replacement items that either one, or both, could both be outside of OEM specs?
Looking at 2 options for now: 1. Attempt to reinstall original tensioner, with the new belt? Or 2. Try to reinstall old belt on new tensioner?
Option 2 will be the quicker action, since the new tensioner is already installed. But not a good solution for long term, as the belt has about 75K miles on it.
Also, I could mount the old and new tensioners in a vice, (one at a time), and compare the amount of max throw/rotation angle, to see if that is the issue. Don't have any way to precisely compare the lengths of the 2 belts, I suppose I could rig something up on a 2x4 stud, mount an old idler pulley on a bolt thru the stud, and pull each belt tight, then mark the board?
Kinda just frustrated that a seemingly simple task is kicking my butt for now.
I will say the engineers at GM probably could not have made a damned belt change more difficult than what it is on this 2015 Duramax. So much crap in the way. If you've done it, you know what I am talking about...

[Ol'Jim]

I apologize for not posting an update after resolving the issue.

It turns out there are multiple GM belt part numbers listed for this truck, due to various combinations of alternators from the factory. These HD Diesels can have 1 or 2 alternators, and several different Amp ratings for each, which all have different size pulleys, so a bunch of different combinations were built.
The counter man at O'Reilly was kind enough to let me look at the details on them, and I found one that was about 20 millimeters longer than the one he had previously sold me, (which I could not install, as detailed in the original post). Swapped out for the longer belt, and it went on just as it should have. So you really need to know how many alternators your truck has, and what the Amp rating is for each of them, when buying a replacement serpentine belt.