Might I suggest first make a close fitting plug to go inside the tubes. Then take two pieces of 1in. square bar stock, one about 18in. long the other 3in. Tack weld or bolt, (two inches apart), the shorter length at one end of the longer and bore between the bolts to the outside diameter of the tube, (plus nothing, minus a thou.) With the plug inside to prevent collapse and the bar clamped on the outside it should shift. Heating the whole assembly will help as the aluminium will expand marginally more with a rise in temperature.
When it comes down to it the tube is replaceable bit so if it is very stubborn . . . !
Finally as this is a job I’m going to have to do in the near future too can anybody say what the threads were please?