Hello John
Welcome aboard!………The sponsons are easy to build, but be very careful when it comes to glueing them in position to the hull. I got mine slightly wrong, by being a bit low.
There doesn't seem to be an accurate logical method of doing this?……Unless it is in the small print?
I would suggest that you build the sponsons as per the plan, including the split joint and study the situation carefully. Fit the paddles to the shaft and offer the sponsons to the hull.
I would position the hull on a flat surface, with access all round and make suitable tempory supports, until you are confident with the setup
Also the paddle boxes need to be considered too, in the equation
Don't rush it, as this is quite a tricky but important part of the build!
Feel free to ask as many questions as you like
Post a few photos of your build, please?
Bob