RE: great planes extra 300 60
I have done one conversion, not for convenience but because it was the easy way to convert a high wing plane to mid wing.
If you are going to run zero dihedral it's easier, but it can still be done with a slight dihedral.
Install an aircraft plywood 1/8" (or a little thicker) on each wing root for reinforcement.
You'll have to make a boring jig to hold your cutter absolutely straight as it extends, and then locate the wing panels (and clamp them) in the correct position for the hole. If you file teeth in one end of your wing tube and set something up to turn it, the tube will work as a cutter. Bend the teeth outward slightly to make the holes large enough for the outer tube to enter. After both wing panels and the fuselage (if not the low wing version) are cut file the teeth off the end of the tube.
Before you glue anything in, do a dry fit of the wing panels to the fuselage, it's a lot easier to make corrections at this stage. When you're happy with the fit, glue it all together.
Now the anti-rotation pin. if you want to get fancy, adjustable mounts are available from several suppliers, or you can just drill through the fuselage to make the hole in one panel, take that one off and put the other on, run your drill through the other way for the second panel.
Bill.