[Arran]

You do have your voltmeter on the right setting? AC volts not DC volts? Between pins 3 and 5 on the 6X5 should be 320 volts AC not 568 volts. Whether it's 115 volts line or 120 line should not make that much difference, normally they used 117 volts as a working number. If they used a 1000 ohm per volt meter that means you would need a 320K resistor in series with one lead on the digital to approximate it, 180K on the cathode.
In any event if there is voltage present where there should be that's what matters. You would be better off just replacing the old caps and checking resistances and continuity rather then voltages. Also make sure that you hook up the filter caps correctly, one has it's negative connected directly to the high voltage center tape, the other to a junction between two voltage divider/bias resistors.
Regards
Arran