Got the rattle fixed, by installing a new M10 capscrew on the AC bracket.
That was after I tapped/chased the threads on the AC bracket bushing. The one by the IP output that takes up the slack and accepts the M10 capscrew. The threads were grody/rusty after 30+ years.
That was after I removed the broken capscrew with a screwdriver.
That was after using a rotary tool and cutoff wheels to slot the bushing and capscrew , which had broken a couple of threads below the flange of the bushing.
That was after I pressed the bushing out of the bracket, in the car, using a long M10 bolt in the opposite threaded hole in the bracket (by the pulleys), plus a helper bolt to drive the bushing.
That was after chasing out the grody/rusty threads in the hole by the pulleys. That took time... not much room for a large tap handle.
That was after removing the new AC compressor.
That was after extracting the alternator and its brackets & idler.
That was after I heard a new kind of rattle... not ones I'm used to hearing.
I'm very happy the capscrew that broke was not the one by the pulleys. That would really suck.
After having done this, if I had to do it again, I'd consider slotting the broken screw in place. Not sure that shortcut would be much easier.