Changing Input and Output Impedance on API 550A Buffer

Well you could add 560 ohms in series with the output, change R6 to 150k, C1 to 560pF,
but then you'd have a -6dB attenuator into 600 ohms.

Buffers are low output impedance so the the voltage gain is unity whatever the load.

You'd need a different circuit to get x2 voltage gain suitable for driving a 600 ohm output resistor into 600 ohm load.