Duly noted thanks.
I'm not big into originality when it comes to clutches, I'd fit a diaphragm clutch, which are lighter and more readily available. Something like an uprated TR6 clutch plate which is also 8.5" might be a good donor. I have an uprated 8.5" MGB clutch on my Bristol engine, it does mean changing the dowels from two to three though.
Those Zephyr flywheels are massive by today's standards, I wonder if not only was the car heavier, but with post war fuels not being so consistent, perhaps the large flywheel smoothed out the odd incomplete combustion event?