Measuring the output impedance of an amplifier does not require technical gear like a VNA, though that will make for a nice quick and accurate measurement. You can also do it using your multi meter and a scope. Here is how:
Connect the output of the amp to the swiper of a 10K pot or better yet, a decade resistor box.
And measure the resultant waveform on the scope.
Starting with the pot set to 10K ohm, adjust the pot until you have 50% of the amplitude of the signal.
Remove the pot from the circuit and measure its resistance, this value is equal to the output impedance of the amplifier.
You can then use a transformer turns calculator to determine the turns ratio for transformer for your circuit. Not hard now is it.