Adrian, these spacers are not super critical and as long as the end dimensions are wide enough to give support, that's fine. The length required will depend on the tank, the later tank has rubber bushes, and over time these will have changed shape from original, so chose a spacer length which does not put any strain on the tank, find a dimension by trial and error. I may be wrong, but I doubt very few of the original spacers survive. The threaded bar trick does the job but upsets the purists.
Swarfy.