I have this new OMC steering wheel made for the Johnson and Evinrude boats of the 60s. I has a splined metal shaft for those steering systems and is all but useless unless one has an OMC boat and unlike ride guide steering wheels it's not practical to make up and adapter.

I want to use it on standard steering systems, either the cable versions, or teleflex / ultraflex..I think they're all the same taper and remember that being 3/8, but could be wrong?? Were they reamed to that taper, or put on a lathe with sufficient clearance (the wheel is 15in diameter)? In the interm have been looking through a huge bowl of borrowed reamers for a match, without success and wondering what to do.

I had a wheel off an old Alfa or Fiat that I used on Bonbon for a while. I drilled out all the splined section leaving enough to screw through to an adaptor bought from www.asap-supplies.com part no. 612600. The plastic adaptor is strong enough that I just used 3 self tapping screws.It will bring the wheel closer to you though. Used it until the leather cover fell off...
They do have other useful steering parts/adaptors that seems to suggest 2 sizes of taper fittings.

It’s probably Morse Taper 2 (MT2), you can buy readers quite easily and they can be mounted in a vice and the wheel rotated carefully to ream out the hole to the correct diameter and depth.