You have a Prova Miura?
There are not many of those about!
Any chance of some pictures?
I presume you are talking about something like this which I have arrowed in the picture. (I realize it's on the steering but it's the only picture I could find on t'internet)
Looks like the hexagonal bit is there just so you can get a spanner onto it to rotate it to adjust the wheel alignment.
Could you not use a round bar threaded at both ends with a nut welded on half way along for the spanner.