Squiring the case is a pretty high possibility.
One other thing I've seen was the bolt that tightens the alt to the slotted adjusting bracket is too long. When its tight, it protrudes into the case too far and stops the armature from spinning. Often times, a second flat washer between the head of the bolt and the adjusting slot is enough to make it work, but a shorter bolt is a better option. Putting a couple washers under the bolt head would give a pretty quick answer as to how much shorter that bolt may need to be.

At any rate, without the belt in place, you need to tighten up both bolts and try to spin the alt. Then loosen one of the two bolts at a time to see which one, or if both bolts are the problem. if its not a bolt problem, the next bet would be a bad bearing, but you should be able to see the pully move in the case if a bearing was the problem. Gene