My opinion - I'ts probably a non-issue if the rivet holes in the side are ok. But if it does need a fix, I think a doubler is the right idea but it needs to be properly tied to the longeron, so make a new one extending further aft say 3-4 rivets worth. I'd say it should be shaped to sit on the bottom side of the longeron flange not sitting on top of the weldment, and have a 1/8" packer where the inside row of holes is on the doubler to deal with the gap.
But first thing I'd do for a structurally significant area like this is ask the engineers at Van's.