There are many different speedo drives (different ratios have different coloured gears) from different models that will fit in the CA18 gearbox. I think the early skylines are the best place to get one from. If you can find a Nissan wrecking yard, they might let you try a few different speedo drive units. I recommend doing this, and using a Bluebird (910) speedo cable (It's 300mm longer than the B110/B210 one).

I've considered buying an autometer electronic speedo, and adapting the electronic drive, but it's too expensive to justify.

Chris.