Not sure if this has been mentioned yet (tl;dr) but last time I did this I loosened up this bolt:



I didn't take it out, and I undid the bottom bolt completely. This allowed the alternator to drop a few inches and the belt went on easily. Also repositioning the bracket after the fact wasn't too hard either, once it got tight the torque from the wrench was more than enough to raise the bracket and put the bottom bolt in.

This however was on my 1980 TD with only one belt, but a larger alternator.