Automotive alternators act like a current amplifier (when turning at a constant rpm) where the output current is proportional to the field current. At a constant applied voltage, the field resistance determines the field current (If = Ef/Rf). If you reduce the field current by putting additional resistance in series with the field, or by reducing the applied voltage, the output current will be reduced proportionally. If you need the full 270A, then you will have to apply the rated field current.