Sorry, I was confused. I read spline and thought finger.

I really don't think A spline is necessary for the joints in a S&G boat. Generally a spline joint is most effective when it is floating and is used to manage expansion in the wood, not to strengthen the joint. If you have a butt joint that you want to reinforce, a simple method is to clamp a small piece of wood under the joint. A scrap of ply will do it, but even that is generally not used to strengthen the joint so much as to ensure that you have a clean transition between the pieces. If the joint is properly glued and clamped in the glue up, even a butt joint is going to be stronger than the surrounding wood.