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I believe the tube is a press fit with a locking agent such as 242 Locktite. With the pump removed you could use a propane torch to heat the tube and secured base then use a tool similar to Vise Grips to secure the tubs and then twist it. Heat the area of the pump near the base, not the tube itself. Then thread as you stated and use a stainless pipe plug.
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Rick
As you slide down the Banister of Life, may the splinters never be pointing the wrong way 
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