67 Barracuda instrument voltage conundrum

I would pull the cluster and do an end-to-end check on the cluster/ IVR/ gauges. you need to rig some test resistors, or if you have an old rheostat or old gas sender you can use that. The test resistances are on the photo here in yelloww

Also do not trust the original cluster grounding which is through the mounting screws. Attach a pigtail to a ground point on the cluster and bolt that to the dash or column support