I know it is a little late but the throw out bearing collars are common discussion in the classic z forums. Nissan actually has several heights for the numerous versions of the FS Nissan transmissions. This is an example of some of the variations which may be helpful.
![](https://www.classiczcars.com/uploads/monthly_2020_07/76521466_DatsunClutchCollars.thumb.png.ad1eb7c21a93af8370727a007136c367.png)