Alumaweld is a compromise solution, so TIG welding is the right way to fix it.
What's the tube diameter and what is in the center? Is that the base of the radiator or some other high stress location?
Alumaweld alone is not likely to result in an effective, long lasting joint, so you would want to add a sleeve around the outside (since there is something inside). Alumaweld the joint, file it smooth, then slide the sleeve and alumaweld that at both ends - if you had to go that route.
Will that thing in the tube melt?