Click on photos to enlarge.
Because C3 thru C6 protruded out the bottom slightly, I had to modify my bottom skin flange just outside of ribs C3 & C6 as you see in the photo above. Basically I cut a small wedge out of the flange so I could get the skin to angle upward towards ribs C1 & C2 on one side and ribs C7 & C8 on the other side.
Other ways to fix the problem were discussed, like tearing out the seat pan & ribs C3 thru C6 and redoing them, but I opted for this fix. I discussed with Peter Beck why ribs C3 thru C6 might have been longer in the rear and he was going to review his forms. There are some trimming involved with these ribs anyway and, had I followed the plans and laid the wings directly on the floor, or placed a flat piece of plywood between the 2x4's I used to raise my structure off the floor, I would have just had to trim the ribs C3 thru C6 from the top side to make the seat pan fit correctly. Had I known what was happening, I would have chosen to do this.
All in all, if I had not mentioned this to you, you would never have known about this mistake as it is on the bottom of the airplane where no one will see it.