Mild Built 340 Timing/tuning help?

100 octane is a waste, not probably. You may even get away with 87. Case in point, my sons 360 is at 10.5-1 with Edelbrock heads and runs on 87 with a smaller cam. (A smaller cams tends to build more cylinder pressure requiring a higher octane.)

Ping … like marbles is a good answer but I’d add that there being violently bounced around until the make a sound like a rattle.