Ralley dash gauges

In the shop manual and in the MTSC.
(Note the 67 Service and Parts manual was released before the 67 Barracuda information could be added, so for most editions thats a supplement)

These are thermal gages.
1. Do a resistance test
There should be around 20 ohms resistance. If its in spec, great, time to see if they work. (For the ammeter, there should be zero resistance. Touch the probes together first - that will be the zero. Physically check ammeter. If the studs are tight and no measurable resistance, then its probably OK.)

2. For the thermal gages. These can be driven with a couple of C or D cell batteries in series. This will reveal if they work. That's a fairly safe check.
Even better yet if they are on the bench, get yourself some 11 and 22 ohm resistors or a variable resistor (potentiometer) and a 12 - 14 volt power supply (a car battery will work fine). Use jumpers to hook up the gages to the resistors. Power the Instrument Voltage regulator in the fuel gage in test.
DO NOT GROUND the gages. Without the resistance in the sending units, the resistance wire in the gages will overheat.

This post will get you further
https://www.forabodiesonly.com/mopa...e-limiter-68-barracuda.522610/post-1973992535
If the IVR is showing steady volts output then someone has installed an electronic version.