I did the alternator on my rear-radiator Cummins B just last April. I had to work from the top. The alternator needed replaced as did the tensioner. The belt would have not been too difficult to change at the point, but it was still in good condition because the Alternator didn't sieze.
I agree with others that it wasn't too much fun holding the alternator into position while inserting the bolt. It was a minute, but felt like an hour. The tensioner was so badly rusted that spring didn't seem to return to its orginal posiiton. The tensioner was almost as much as the alternator.
Bring the old tensioner with you to the parts store, because I orginally got a Cummins tensioner that worked in the opposite direction. Sending the alternator out for a rebuild may take a while. My NAPA had the alternator in stock, but the (correct) tensioner was a special order. Hopefully your tensioner is still in good shape.
After freeing the alternator from the block, I didn't have to remove any other hoses. I was able to find a clear pathway out along the frame almost 3 feet back. Just remember how you got it out, so that you'll know the way back.
