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Ok, the values shown on the schematic are the DC resistance of the transformer windings, not the impedance..(the 7k and 4 ohms) You can get close to the Z of a speaker with a resistance measurement.
Knowing the load impedance that the output tube wants to see, you can determine the turns ratio needed. So, in this case you need a "Z" (Impedance) ratio of 1750:1, or a "turns ratio" of about 42 to 1. (sqrt of Z).
How does this help? Taking an unknown output transformer, and injecting 1VAC on the secondary, read the AC across the primary. Something close to 42VAC and you're there.
It doesn't have to be super precise. What I use for getting the 1VAC is a small 6v transformer run on a variac.
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