Trying to visualize.
From the picture the top of the rudder post is something like 6" off the hull. Do you want a straight arm (parallel to the hull/perpendicular to the rudder post) that is 6" off of the hull (ie you will be sitting wherever that picture is taken from with a tiller in your hand but very limited visibility) or can you go stright up off the top of the rudder post (and or associated hardware) and get above deck so the person stearing can be up on deck.
Look again at the pitcure from the JC. That looks like a stright shot down (probably 3-4') through the transom coaming board (maybe it is through the deck and only 2' down but the same issue). Is your rudder post under the board/deck? Can you get access straight down at it?
Now for the analysis: Assuming you want to run the tiller from up on deck you have two options.
1) You put what I will call "the riser" (part that runs from the top of the post to your rudder) directly over the rudder post. The benefit here is that the turning radius of the riser does not need to be more than the diameter of the riser. Problem is your access port needs to be directly over the rudder post and that may not be practical.
2) Alternatively, you put the riser into that open hole at the front of the steering quadrant (6" in front of the center of teh rudder post in your picture). Now the turning radius of the riser is the swing arc that is 6" from the post (you need, at the very least, a C shaped cut in the deck that is 6" from the top of the rudder post).
I have spent enough time on sailboat tillers to know that from the perspective of the attachment/rudder post: whether you go out, up and then out again or you go up and then out either can work fine and will not adjust the loads.
Once that is cleared up we will know where we need to attach and be able to work on how to attach it.
I hope this helps. Words are confusing but the picture is clear in my head (which, of course, does you no good).