Answer: My research shows that Cyathea cooperi is intolerant of topping so reducing the length of the trunk down to 3' will result in death of the remaining trunk. It may, however, generate a new trunk from the roots, as is what often happens when an Australian tree fern is killed down to ground level by a heavy freeze. I think I'd move the plant rather than try to control its size.
Best wishes with your landscape.
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