After using my new autopilot for a season I figured out how to install the rudder position sensor. Theres no room for the arm to swing and theres no tiller arm for attachment. I mounted the sensor backwards. The body moves and the arm remains stationary.
Because the sensor arm has less travel than the rudder I used a bungee cord to hold the arm in the middle. When the sensor arm reaches the end of its range the bungee streches. I expect to replace the cord when needed.
The sensor reads backwards on the screen so the red and green wires must be reversed. The ST4000 has a small calibration range so the bungee cord does not need to be perfectly centered. I was able to adjust the base initially thereby saving the full calibration range for later adjustment should the bungee get a little tired.
When the floor board is back in place I have 1/2 clearance. The wire is routed up the arm and has enough slack for arm movement but not enough to foul steering cables. I think this wire warrants careful observation for wear and may need a protective sleeve.
By the way, while working in this area I discovered the steering cable idler was not always turning and the cable had worn a flat spot, also the double nutted steering cables were not locked and the cable was about to come off. Since owning this Catalina I have discovered numerous, numerous, items not tight. I have lost some respect for the builder and wonder if they can be trusted.

ST4000 rudder sensor

Notice the bungee cord under enough tension to keep sensor arm in middle.

Sensor and rudder going straight

I used a small chunk of unistrut angle hammered to proper angle and attached to rudder post bolt. Any piece of metal would work here so long as you can bend it into proper shape. Theres just barely enough room under the floor board to fit rudder sensor so forming metal is a must. To keep away some rust temporarily I sprayed it liberally with Galvinizing paint. You can see the three little holes in a triangle shape matching the sensor base slots. I used self drilling screws and cut off the excess length. The sensor base has slotted holes allowing for some final adjustment. The sensor arm needs to point where the rudder points when your all done.

rudder sensor ready to attach

Notice the bungee cord has allowed sensor arm to move beyond the 30 degree design range without damage. You can also see the wire is hanging loose at this point. I wish I had a better way to route wire

hard to port

Notice bungee cord has gone slack because sensor arm has moved past maximum range.

Hard to starboard