Well, I'm surprised no one's mentioned it yet but it's quite common to have tapered Aluminum masts that are purposely cut and welded back to provide the taper. Admittedly this is common at the top of the mast (like on the 30' Sori wingmast I have on my old woodie A-cat from the 80's) where the loads may be lower but that also needs to be the most flexible and still be strong. Don't know where you are but you might try looking for a welder familiar with that technique (there are also some good marine grade welders who fabricate the fishing towers for powerboats or aluminum frames for tops, also some good welders who fabricate/repair the expensive bikes- I know, they are all CF now but the skills should still be around) - Just a thought-

Kirt