I'm no expert engine builder but I've done my share.
Lately I've found one has to remeasure and recheck everything the machine shop does. The trend seems to be more clearance in the mains. The guy who machined mine bored the mainline for 3.5 thou., even though I told him I wanted stock clearances and I'm building a street car not a race car. He's a Ford guy and builds drag race engines. He had to re-do the mains and in the process ground the caps at an angle other than 90 deg. That was reason for yet another mainline bore. I found three other vital mistakes after I did all my pre-assembly inspections. Obviously I have another machinist. Sorry to get off topic.
Some debris may have been in your oil. If the block and all the components are not thoroughly cleaned any dirt will cause wear. Of course the worn off metal will accelerate more wear. The whole works should be washed with soap and water before it is assembled even if the machine shop says it's clean and ready to go.
The journals on that camshaft appear to be a little beat-up. Old engine blocks can settle and slightly change shape with 40 years of use and countless cycles of heating and cooling. Pontiac blocks have been known to settle enough to have a cam tunnel not quite true. Be sure to check the cam tunnel for straightness and roundness. I measured all my cam bores, they were round, but when I installed the camshaft it would not turn freely, The tunnel was not true.
Your pistons may have had the correct clearances but what about the rings? Not only gap but ring side clearance and depth as well. If the rings are too tight in the cylinder it could cause wear.
Did you check lifter to bore clearance?
As far as oil pressure and clearance, some say to increase the main clearance to get a larger oil wedge between the bearing and journal and more oil flow between the bearings and journals may lower oil and operating temps. I think a standard Melling pump and a 2-1/2 thou. clearance will give you enough oil and still have preasure at the red light on 95 degree weather.
Sorry again, I'm running off at the mouth.
If your pin is loose in the rod and came in contact with the cylinder wall it could have been the root to all your problems.
