CLose call V belt ruined

66 Dart \6. Freshly rebuilt engine, new belts. Finally got her on the freeway, got up to 65mph for 10 minutes. Seemed like it was doing good. Drove it around town for about 30 minutes.

I had just removed and reinstalled the Alt. Got it tight.

This morning I look under the hood, and I see the Alt v belt has flipped over and is sitting sideways in the groove. The belt is seconds away from breaking, cracked everywhere worn real good, and loose.
Any idea on what would cause this? The belt was under warranty so I got a new one, but this is a cheap belt. When i removed it I noticed that the pulley on the Alt. spins a little warped, not completely straight as it turns. Meaning it is not true as it spins. help!!

The new belt feels sorta thin. There is an option at Oreillys to buy a Gates belt which is about $15 more than this belt.