1997 Ford F-150 - Alternator issues

Hi, I have a Ford F150. Sticker says 4 of 96 but VIN and body are a 97, so I am going with 97.
4.6 with A/C
My Alt went bad so bought a new one, removed the belt, put on new ALT, using the belt guide sticker on radiator shield I routed belt one but does not fit! % people later, all looking say yes it is routed correctly, but again is too short to fit.
The belt number does check out for this engine with A/C
We were able to get it routed but the groove side of the belt is resting on top of the idler, it should be smooth side underneath the idler.
it seems to be the right tension and the truck runs but the belt is not routed properly, it is a mystery!!
I will say there were faint groove lines on the idler, as if the belt was running the groove side like I have it but I never looked when removing it, I mean I dont think I ever have on any vehicle!
Any help as to why this is would be much appreciated!!!

Did you compare the old alternator and the new one for exact size, fit and pulley size? Clearly something is different. Since only you have the truck and the parts in front of you, only you can make that determination.

If I have to take a blind stab at it… the pulley is a different size on the new alternator than the old one. OR, you aren’t moving the tensioner enough mount the belt.

Hi,
Yes the ALT is exact along with pulley, The tensioner was pulled all the way to full stop position I know this is a mystery but really trying to solve it!!

Did you replace the belt or are you using the old one?

using the old one, and sadly did not look how it was on before I replaced ALT

Tester