I agree with much of the advice given by others. Also, consider heating the iron base (and indirectly the aluminum tube), then applying something like circuit freeze to the inside of the tube. The idea is to drive the maximum amount of thermal expansion difference between the parts. The high thermal conductivity of the aluminum should help here. Combine this with the penetrating oil and run multiple thermal cycles.
There may be so much galvanic corrosion in the threads that they may not come apart without damage. This may be a case where it is better to just leave them assembled.
If you are obsessed with getting it apart and all else fails, the tubes could be cut off and everything rethreaded. The stands would be a bit shorter, but you would be able to reset the clock on the corrosion cycle ;-).
Glitch
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