1968 Barracuda gauge issues

There's an MTSC booklet to go withthe filmstrip. Look through the mymopar collection for 1966 and years close to it for session 227.
Imperialclub's collection has a lot of broken links at this time. :(

First test I suggest is check each gage's resistance.
Gauge Cluster Issues/IVR

If you set up a live test of the IVR on a bench the output voltage will look something like this on a 0 to 8 V scale.
Gauge Cluster Issues/IVR
Scroll up in that thread to see the resistors used to simulate the sending units.

A lot of good info in this thread as well. Redfish knows this stuff.
Bench testing rally gauges