Jerry,
This has been done many times. Look at any Ziroli 3 pieces wing design. Basically, you go to the last rib past the retracts. Cut the wing. Cap the rib with plywood, Make a ply rib for the wing you cut off. Put pegs on the wing tip and hole for the pegs on the other rib. Connect the the wing. Open the under side of the wing 6" either side of the cut. Get a 12" wing tube and sleeve. Glue in the sleeve on the back side of the spar or spar box. After it's cured. Cut the sleeve where the wings join. close up back side of the wing. You will have glue the wing tube in one side of the other. On the opposite side you will have to drill and tap a hole thru the wing into the wing tube to secure the wing.
THE IS A OVER SIMPLIFICATION! But it gives you the basic Idea. On most 80" wings you only have less then 20" past the retracts, most on the stress on the wing happens from retract to retract. The wing tips will handle the stress with no problem.