If you can't modify the boat, then you are stuck with rotating the stock
I think if you are going to use a worm drive then you are best using it with the twist grip tillers, putting rope onto that system would be tricky.
if you 100% want the rope adjustment then I would use a tube for the custom plate, with a hole at the bottom and small slot at the top, it will locate better.
Then dead end the string on the tiller, a single block on the "custom plate" fwd to block on the tiller. The line is then split into a loop, one side of the loop down each side of the tiller to one block each side at the top of the stock, then down to one block each side at the bottom of the stock close to the pintle. You then put a floating block into the loop which is attached to the rest of your purchase.
The loop and floating block mean that there is no change in the length of the line as you turn the rudder.
Born Slippy's description sounds good too