I corrected my post a bit, and added a link to the NTSB report. I’d thought the 172 was IFR, but technically it was VFR, doing practice instrument approaches “under the control of San Diego Approach Control” in VFR conditions.
The 172 was climbing, the 727 was in a descending turn.
Ugly all around. Some TCAs (now Class B areas) had been established at that time, but San Diego did not have one yet. A San Diego TCA was soon established in response to this accident. Of course, under the facts of this accident, a TCA/Class B would not have made any difference.