Of course the owner wants it to be as original as possible or at least
retrievable. I've convinced him that a fully bonded earth point to the
chassis is absolutely necessary for use in the UK, no getting away from
that.
Hopefully the US mains cable rubber insulation will be perished and will
need replacing anyway so would then be 3 core.
He definitely does not want the step down transformer mounted in the cab,
but external .
But if original US mains cable and US plug retained.
I'm thinking earthing coloured multistrand flexible third cable run
alongside the US cable with black heatshrink over the near-enough whole
length. The problem is what connector to use for reliable earth continuity
to the step down Tx and UK mains cable and how to make a mechanical
interlock, in effect, so the amp cannot be run without the third wire
connected.
Then what to do with the original "ground" switch ? presumably disconnect
but leave in place internally .
Do US three pin variant outlet, purpose made, step down transformers exist
in the UK, I only ever see 2 pin outlet ones. Assuming he can tolerate
swapping, but retaining with the amp, the ancient 2 pin plug.