To not have to reshape the saddle:

make the center appx 6 inches of the new wing flat bottom, same airfoil as the current wing. At the transition line on each side, use a 1/4 inch thick rib. You will have to cut rectangular holes in the ribs for the lower spar, to keep it in the same place. (needed due to the center section)

You could build an identical to original wing and add small bottom sections to the ribs AFTER the panels are joined (but before the bottom's sheeted if they have LE sheeting over full span...) at the reduced dihedral. Then add a 1/8 inch spruce stringer below the spar... just to aid in maintaining the airfoil when the covering shrinks.

There's several MORE approaches for this.