, what a great adaptation and excellent presentation! Your quality and finish are first rate.
For the rudder mount, it looks like the critical part here is your welded extension, which was very neatly done. It appears that the spacer is at a slight angle for the rudder to clear the boat, and the bottom portion of the original rudder mount is shortened for the same reason.
To simplify, could you keep your spacer parallel using an off the shelf 1 or 1.5 " (3 cm +/-) bar-stock and instead, cut a notch in the leading edge of the rudder so that it clears the boat (and keep the original down-tube, providing a little more down leverage)? Here is a poor example of notching the rudder:
These rudders are real easy to re-shape and customize.
With your merger of the two rudder systems, are there now two ways for the rudder to kick up -- the Twist N Stow kick-up system and the old kick up method?
Your line management scheme is brilliant, handling both rudder deployment methods at will! Again, everything looks professionally done. IMO, this has the potential to make a nice little conversion kit, and opens up access for any of the older models to the sailing rudder.