I would probably disconnect 12 volts from one of the RCA accessory jacks and run a wire to the vfo signal on the mixer board.  You could solder right to the connector pin.   The argonaut has a 9 mhz IF.  The vfo is 9 mhz above the band frequency on 80 and 40 meters and 9 mhz below the band frequency on 20 15 and 10.