I ran into the same belly pan problem, wound up sanding a bit off the trailing edge. I still had a small gap along the wing, so I sanded about a 1/2 inch strip over the joint, used some filler putty & painted the seam. I few extra steps but easier than trying to get that thing to fit then magically get epoxy only where I wanted it.
Using Hitec HS-635's on all surfaces. Thinking of a Saito 182 twin for this one; any other twin powered Gee Bees out there??