I installed an autometer temp gauge on my '89 XJ.
1. replace thermostat housing with a 92 or newer one that has the threaded fitting for the fan temp sensor.
2. autometer has an adapter to install it's sender in this fitting, if using a mechanical gauge, you will need to get a pipe fitting to add about 3/4" more depth. (the thermo housing is normal pipe thread as is the autometer senders) just take both to the hardware store and they can help you.
3. don't forget to wrap the fittings with teflon tape or use pipe dope!!