clearly the outside 2.5" either side of the cut is exactly the same as each 2.5" of the extension piece, so the unknown profile is the inside, as you say the Extension piece is thicker, so making the outside flush is more important as this should match the hull, bar the slight inconsistency as the hull curves, the scarf piece although is good as a strengthener, its going to pull your hull in at their joints, both sides of the join needed to be identical in thickness, something you could have achieved by packing out the hull side till it matched the extension piece thickness, did you account for that?