Looking at the photos you have posted I see that you have deleted the A/C (or you have never had it). Check that you have installed the correct length serpentine belt.
Have you checked the indicator position when you installed the belt? It is kind of a strange that the belt will stretch out of the indicator range. If that is the case buy better quality belt (I personally use Gates).
You can also check the spring tightness of your belt tensioner with breaker bar and socket, when you slip off the belt. If your tensioner spring is loose, then the indicator will point towards the minimum belt length (will not be able to keep the belt tight).
BTW, your engine bay looks really nice. I got confused from the alternator reflection that you have auxiliary serpentine belt.