Hello, my name is Jake and I work for Nordskog in California. We saw a few hits coming from the UK, so I figured I'd check it out. By the way, nice work on the instrument cluster, very clean.
If you have any questions about the gauges, I can help with that (jake at nordskogperformance dot net). When it comes to vehicle specific issues, well, honestly you know more about 300 Z's than I do.
Regarding the A/F gauges, we make a matching unit that works with Innovate Motorsport's LC-1 and LM-1 devices. We sell lambda versions to a company in Germany also. We're still working on a couple smaller retailers in the UK. http://www.lm-1.de/contents/en-us/image_viewer.html?lmd=39842.350694
And regarding the speedometer, we do have a way to get a speed signal that doesn't require messing with the transmission sensor or ECU, it uses GPS, and it updates 5x/second.
http://www.nordskogperformance.net/product.php?productid=19231&cat=0&page=1
It's setup for 8000 or 16000 pulses per mile, but the speedometer can be calibrated to convert that to kilometers.
Regards, Jake