Looked at it some more, bigG is right... the tube needs to be longer. The ending point of the stock tube is just too close to the center section, where most of the wing stresses are located. A longer tube and sleeve, with a simple box structure built around it, should do the trick. Maybe even carry the center section sheeting out a bit farther.
I've had the urge to build a kit for awhile, so I guess this project will help me relive the good ol' days.