Hi Jay,
I just tackled the problem of a leaking steering box in my 61 MK IX. It has leaked for many years but I am having my garage floor refinished and it was time to finally fix that leak. I looked over the exploded view drawings in the manuals and determined my box was leaking on the main shaft (rocker shaft) as it exits the bottom of the box. Further study showed that it can easily be removed by removing the drop arm (requires box be removed, a puller and patience) and then unbolting top plate and tapping the shaft upward with a lead hammer. The shaft had been scored and the two o-rings were hard as a rock. I bought an o-ring kit and sent the shaft out to be chromed and ground back to slightly over standard (1.124") dimension. Before and after pictures follow below.
I will now have my machine shop hone the original bushing in the housing approximately .001" to fit the rocker shaft and reassemble with new o-rings.
And to answer the original question, I have run radials on my IX’s for 21 years with no issues and enjoy the increased handling ability and ride that they provide.
Tom Brady