You can notch the back side of the leading edge [no more than 1/8" deep] with a razor saw to slip fit the rib.

The biggest problem with this plane is the enormous amount of lead required to get the CG.
I've seen a pound of lead being used. This is ridiculous for a model of this size. Might as well build the whole forward section out of pine 2x4s. I never have understood why those who build this plane don't move the engine forward. The net result will make a big difference in how the plane handles.
Over the years I've seen 3 NorthStars show up at the field, [lead and all] and all 3 had very brief life spans.