If it helps I extended my MX5 column to do away with the extra larger UJ.
I bought 2 steering shafts from Rally design. 1 I cut and sleeved with 20mm thick wall CDS to make the lower shaft. The other I used to extend the MX5 shaft. This was done by a local machine shop. They drilled and threaded the mx5 column, then threaded the shaft and screwed it into the MX5 shaft then tigged it.
