First and foremost, make ABSOLUTE CERTAIN that your flywheel is NOT LOOSE. They can feel tight, and still wander, and sometimes they'll develop a totally invisible internal crack, that under the right conditions, will expand and release force in the taper. In MOST cases, the cast iron flywheel will stay together fine under 'normal' engine governance, but I've had one that had an invisible crack open up at a 'slightly above factory governed speed' once, and I caught it before it came apart...
The K301 had balance weights (balance gears) that spun opposite to crankshaft direction to counteract primary imbalance. They need to be shimmed and timed properly on assembly, and if they've got wear, they'll be sloppy. When these gears aren't done right... and sometimes when they are, the'll come unglued and go on a lateral vacation, leaving a picture window in the block. Usually, the warning is a rattle or knock at low speed/partial load or idle.
Most likely NOT spark knock, although these flatheads WILL, given enough of a load. air-cooled flatheads develop hot spots pretty easily... but what you describe just doesn't sound like preignition to me.
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