The "quickie" test for either oil or temp is to ground the sender wire and turn the key to "run." The oil light should be active, and the temp gauge should head quickly to hot. Don't leave it there for long
I cannot remember what a 68 Val dash cluster looks like "behind." It might be there's an interconnect between two sections, etc
Switched ignition power goes to the cluster, where it powers the IVR/ limiter. From there power goes to the fuel and temp gauge, through the gauges, and out their respective sender wires to the senders, which produce a variable resistance to ground
You have a shop manual?
