I converted my 54 Ford Wagon to 12 volt. Using what I think is a stock Ford instrument voltage regulator. I have two of these regulators, slightly different style but both behave in the same manner. , I get two issues.  Measuring voltage in is 12 and out is 11. 9. The wiring is hooked up correctly. When I remove the wire to the gages/output the voltage goes up to 12.
The other issue is when I mount the regulator to the metal above the kick panel the idiot lights flash and the fuel and temp gauge go to off.  When I pull the regulator away or let it dangle the idiot lights, fuel/temp gauges seem to work ok bu the idiot lights seem brighter then I remember. In looking at a 57 Ford Manual the regulator is mounted to the rear of the speedo pod. Not sure that is significant.