If you have rack and pinion steering with solid rods then I would guess that a linear drive onto the quadrant would be the right choice.
Some years ago I had a Moody with a linear drive direct to the quadrant and it saved the day when a swage on the steering cables let go and the wheel spun freely in my hands! This was 30 miles south of Bishops Rock. The emergency tiller connected to the rudder shaft in the aft cabin and you had to stand on the bunk looking through the hatch with the tiller between your knees – hopeless in a seaway. We steered 40 miles to Falmouth and even berthed using the autopilot. A rotary drive would not have helped in that situation.
What you really need is a good look at a professional installation on a 39 to see how it’s done.
I’m afraid I can’t help any further.
Good luck
