Create a boundary surface. Only use direction 1. Select the edge of the existing fairing where it ends, touching the cabin. That will be one contour. Then select the edge of the wing where you want it to end. This will be the second contour. Set both end conditions to "curvature continuous". You may have to press the button with two arrows to switch the direction on the first curve. You want it to match the existing surface. This should create a nice smooth transition from one to the other.
If you can live with the intersection between wing and fairing lying in a plane, then you can put a plane at the intersection and then use Intersection Curves to define the edges. If you can't live with a planar intersection, then you could build a surface more or less perpendicular to the wing that gives you the intersection that you want and use that surface to trim the wing. Thinking about it, even if you want a planar intersection, you will probably want to trim the wing so that you can use tangent to surface conditions, as Charles suggested. If the wing actually passes through the fuselage, you may need to use offset surfaces and trim them.