That alternator bolt is correct. 7/8" long with 3/4" cap washer. LG head is another common example. If you look at the alternator there is only so much thread in the front cover. So 7/8" plenty long. If you stripped your cover you need to fix that. Cover is only thick as nut so a longer bolt does no good plus gets bolt closer to rotor.

If it is not the belt, what about the top bolt going into the head. Many time they maybe too short with only a few threads engaged. That play an important part in locking up the alternator via the sleeves. Loosening the alternator bolt without loosening that bolt and my alternator doesn't pivot.