I used this one:
12V/24V 52MM UNIVERSAL TACHO/REV COUNTER. 0-4000RPM. Original Durite. | eBay
Its a direct replacement for one of the smaller dash gauges, the only thing is it sits a proud of the hole and not flush like the current gauges, I have replaced my temperature and fuel gauge for similar ones so it matches a bit better (plus my fuel gauge stopped and we all know the temp issues). At night they are very bright and really overshadow the speedo although I am looking into upgrading the back lighting on the speedo.
They are easy to fit, you just connect to the W terminal on the alternator, you have an adjustment on the back of the gauge for different alternator outputs. It is so simple to wi:decision:re in as there is a hold directly behind the dash, the rest utalises the current wires.