360 LA getting oil in #3 cylinder.

Definitely detonation, the cylinder pressures would have been astronomical.
Those marks aren't good but not much you can do in the car..
Detonation likely caused the pressure to push
that gskt out.
As mentioned, retard the timing or whatever it takes to stop detonation.

http://www.contactmagazine.com/Issue54/EngineBasics.html