I had the same problem on an old big block that I ran in my tow truck.. I replaced the bolt through the slide bracket with one that threaded all the way through the ear on the alternator. After that, when I adjusted the belt, which became a seldom needed task, I tightened the slider bolt. tightened the lower bolt and then tightened a nut on the back end of the slider bolt. I discovered that the bolt in the ear of the alternator was loosening during operation and the "double nut" kept it from doing so.
I also added a hardened washer under the head of the adjuster bolt and one under the double nut. The factory washer was too soft and deformed over time and repeated adjustment.
