WALBRO Pump for Carbureted System??

You can test the error of the liquid-filled gage at temperature by turning the car off and seeing the "zero reading". You may have to vent a hose to get true 0 psig, depending on the pressure regulating system you use. Even if it does have a major temperature drift, use it while cold to set your regulated pressure, then don't worry about the reading when warm. The regulator should hold the setpoint.