Duncan, in short you make up a simple reduction drive, overdriven in this case.You need a steering shaft end with the same spline as your current shaft so you don't have to change stuff there.
I use a piece of tube turned a little inside at each end to make an internal step to accept the appropriate bearings that take the cut down 2nd steering shaft with a flange welded or circlip'ed one end, and inner bearing spacer tube.
The 2 sprockets are cheap industrial type welded on, as is the chain industrial.
I then weld 2 5/8ths or 16mm (whatever) bolts to the tube.
A simple plate with 2 holes to accept those 2 bolts with a nut either side of the plate to enable me to adjust the chain tension. Another shorter gusset plate to make triangulation, all welded on to the master column (be aware of nylon bushes).
I have done this 4 times to my own cars over the years and helped Mates and others to do it - never any trouble but it is your steering so over engineer it for your own sake.
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