I would use epoxy (not five min.) and you should sand off any anodizing and use a surface prep. ie. acid wash.
If you weld you will lose a lot of the strength in the mast.
The sleeve I have used are about ten inches long.
I agree. I would not weld a mast unless the builder agreed.
Keep in mind there is a lot of compression load on the mast at this point. Make sure the external splice to mast joints are square. The parts will be pressing against each other with 100's to 1000's of lbs of force.
I once saw a 18 square mast with a splice like you are talking about. The splice kept breaking so the owner kept beefing up the splice. Finally he had a 3 ft internal splice and it held. I looked at it a couple of years later and the metal at the splice had deformed where it was pressing.
