Troubleshooting problems with temperature gauge

Since you have oil, that means you have a Ralley cluster, and that the IVR is inside the fuel gauge. Since oil and fuel work, that means the IVR is at least "working" if not accurate. Power for the T gauge comes off the cluster common to the fuel/ oil gauge and runs to one stud of the temp, through the gauge, out to the harness connector, through the bulkhead connector, and to the sender

The possibilities in no probable order are:

Break in the wire at the temp gauge connector or corrosion/ looseness there
Bad connection at the bulkhead connector terminal
Bad connection at the Cluster PC board wiring connector / pins
Bad connection at either of the temp guage studs
Bad temp gauge.

Likely your next project is removing the cluster.
You have a service manual? Go to MyMopar, download one free. The do not have 76, only up through 73. The wiring is quite different, but the mechanics are very similar.
Disconnect battery ground!!!!