Since this appears to have happened while you were cranking the motor, the damage to the motor's internals may not be terminal (no holes in the block or head). A complete teardown is necessary however, to assess the condition of all parts, the crank, the connecting rod, etc.
A note for everyone else,
This is a classic example of what happens when you run the stock two piece valves, its a ticking time bomb in each persons car. This happened to ht4tchr in his driveway, while starting the car, NOT at high rpm.
The previous owner had this motor apart. If he had spoken with somebody who knows Clevelands, he would have been instructed to replace the oem valves. Lecture over.
George
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