Before I crawl under the dash
How does the cluster and dash usually ground? There's new hardware for all the spots where the dash bolts on to the frame. The cluster screws to the dash through the plastic Bezel, is there a ground wire somewhere
67 cluster is grounded by the screws that hold it to the metal dash. I believe later models (don't know when) changed that.
Technically you could just take a volt meter and clip to the 5-6 volt side a gauge post and look for a fluctuating voltage around 5-6 volts.
You can VERY temporarily like 1 to 2 seconds, ground the temp sender wire and look for guage movement.
In all reality if you rewired the car there are too many posible points of failure to count.
Basically you need to trace out the wires from end to end.
ALL THAT BEING SAID...
If both gauges fail it is either.
1. input voltage to the IVR
2. Output voltage from the IVR
3. Grounds
4. miswiring
5. 2 bad gauges (not likely)
6 2 bad senders (not likely)
Lastly.
If you have to crawl under the dash remove the front seat / seats 4 to 8 nuts and you will thank me!