I would have  a lot of concern about doing this welding on the airplane and remember once doing some longeron cracks which were fixed by welding .The strength  and integity of the weld required an internal doubler of several inches each side of the crack. We drilled holes along the repair as well to weld for additional strength.I think they called them rosette weld points.  Also the heat required  was intense as the whole member had to be brought up to temp and tempered as only a true professional can do. Also just because it cracked at all i think reinforcment would be a must with doubler internally and gusseting as well. I would address these concerns as evaluate the TIG welding for overall repair of this lmportance. On my longeron repairs, the engine  had to come off and I had to internally drill the steel tube for exact fit of the doubler.