I'd have to see what the original sending unit looks like to give best information since I'm not familiar with the 57 models. If it had King-Seeley instruments then they were a current change based system instead of the resistance based systems that have been used in the modern era. To the best of my knowledge, there is no source for KS type sending units. The indicator gauges were manufactured to work on the current provided by the sending unit. When things went to 12-volt, they started using the pulsing current type constant voltage regulators to lower and stabilize the current draw of the system in order to keep current low which also affected the voltage according to OHMs law.
The modern replacement sender units are all rheostat devices that vary the current/voltage between the sender and the indicator. The indicators are calibrated for a different type of system so they don't always work to give good indications and many times it just can't be accurately compensated for the differences in the two devices.
There may be a problem with the constant voltage regulator too. It should be constant no matter what the configuration of the components is. The solid state CVRs are usually better than the old bi-metallic pulse types but I don't know what you have there.
I'd either look for a functional original type sending unit or I'd change the indicator and sending unit with compatible OHM value types. There are near a half dozen different Ohmic values currently available in sending units depending on the make & model of the system but there are also aftermarket systems available. Aftermarket set ups may not have a indicator that can be retrofitted to your application so it would take some research to find stuff that might work.
