couple other basics might apply here---------sealant on sender could isolate grounding thru threads------

Truck originly was 6 volt positive ground----now 12 volt will be negative ground--------

instrument panels mid 50s had resistor in feed to gauges

If you have a lazer pointing thermometer check the temp patterns of the whole radiator which will show flow patterns of the water and any cooling effects plus possibilities of runner bloakage--------

Do you have correct rotation water pump ???????? are head gaskets on correctly????????