OK I had O'Reilly's order me a Gates XL 9525. There is no such thing as 8525 any more. The 9525 did not fit, it was way too wide. So I took it back and had them give me a 7520, which is the exact same one I was using before. I put it on. As you can see, it substantially fills the groove and it is noticeably wider than the squeaky 7520 I am replacing. In this picture I held the old belt in the next groove of the water pump pulley for comparison.
I do not remember the old 7520 belt riding high in the alternator pulley when I first installed it, but I suppose it could have been that way and I was not paying attention, and perhaps that causes the belt to wear and get narrow. I suppose I will need to try one of those deep groove pulleys but I don't know if they are compatible with the 7 series belts. If it is, then I will need to get a longer belt because this one is going to be too short to fit over the pulley. Then I will get into interference and rubbing on the power steering pump, and very limited range of adjustment. I don't know if a 1972 upper alternator bracket would allow me to really use a wider range of adjustment without the belt rubbing on the power steering pump.
The stock alternator has a much wider range of adjustment because there is a big indentation in the case that allows the valve cover it fit in it. This alternator is a CS144 which does not have that indentation and is also an inch bigger. What I would really like is an alternator lower mount bracket that holds the lower alternator out 1 more inch away from the valve cover. Then I would have better range of adjustment.
The problem is if I get a belt that barely slips over the pulley when the alternator is adjusted all the way in, by the time I adjust the belt the alternator is already at the all-the-way out position and belt is too loose. So I need to get a short belt, and remove the lower alternator bracket and rotate the whole bracket and alternator down just to put the belt on. There is no way for me to win it seems.
Overall, I'm pretty bummed out. I just want this belt crap all figured out. If anybody has a GM alternator pulley that they know works with the 7 series of Gates belts, fills the groove but does not result in the belt riding high, I would be happy to buy it and pay postage.