hi paul,
if you already have the bottom pulley on your engine and you want to convert to run power steering as well all you need is the power steering pulley which will bolt on to your existing pulley. there are three nuts and bolts holding it together, remove the half shaped circle piece that is on the end of your existing pulley and replace with the power steering pulley. i do have one if you are interested. which special nut do you mean on the power steering box.
ian