Thanks for the thread. Biggest thing it revealed is that electrical gremlins can be a bear.

At this point I have a theory:

Original sending unit disintegrated. ( rust )

Voltage reducer failed, sending full voltage to guages.

Oil guage isn't connected, so it was unaffected. Temp guage works, but reads low. New tank sending unit melted from the 12 V.

Stock voltage reducer operates above 5 V, so the electronic gizmo I installed isn't powerful enough to operate the gauges.