You're not aligning the wing with the fuselage. To do this, you should use a piece of cotton string or a stiff wire. Put a pin into the fuselage at the centerline of the tail. then use that to set the distance to the rear corner of the wing. Adjust the wing as necessary until the distance is the same on both sides. If necessary sand the wing saddle to allow the wing to sit properly when adjsted. If there is some small misalignment between the fuselage sides, you'll not likely notice it when flying the plane.

Do not cut the fuselage.

Brad