Start with easiest which the coolant temp gauge, check thermistor values change with a VOM (volt/ohm meter) as temp changes, check right at sensor and if working trace back to cluster wiring and check regulated voltage feed for the gauges, the sensors work on the ground side .
I'll see if I can link to some wiring diagrams .
The regulated control and the clocks can all be removed/replaced
Both fuel and temp gauge share regulated input voltage .
If you not had clocks out already you want get them out and have good look at wiring as lot been messed/spliced into which leads to issues .