Wouldn't use threaded bar - entirely the wrong thing to use.
You need a length of 3/8 605M36T and a 3/8 BSF die and stock
Alternatively Terry Foxen at GEE will be able to help as he supplies them with the new crossheads he has made for me
Kind regards
Arwyn
Appreciate the advice but considering the "proper" bar snapped around 14,000 miles and my version has now done well over 54,000 miles, I know which one I will stick with.