If the output transformer is wired to the impedance selector switch, you will see continuity between the various switch lugs and to ground. But you are just seeing the secondary windings on the OT. These have very low DC resistance, maybe an ohm or less, so they can easily be mistaken for a short.
If your meter can read these low resistances, measure across the tip and sleeve of the output jack. You should see the lowest reading when the impedance selector switch is on the 4-ohm setting. As I said, this might only be a fraction of an ohm, but it shouldn't read 0.
Switching to the 8-ohm setting should give a slightly higher reading, and the 16-ohm setting should be a bit higher still.
If your meter can't tell the difference between, say, half an ohm and a short, you will have to disconnect the OT secondary leads to test the impedance switch. It's unlikely that the switch is defective, though.
Good luck with your build!
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