Yep that's correct MMC in the handbook for classics quote for copper cored plugs 1.25-1.35mm. The original thought with iridium was to be able to carry an increased gap size as the smaller precision electrode allowed the gap to be jumped more accurately, it could leave the electrode more easily and from a predictable point. The plug design allowed for a fatter spark to develop and therefore ignition was more efficient but as has now become known this brings problems to even iridium electrodes and coils and coil packs. This resulted in a period of several years to resolve and cost Ford a fair bit in coil packs. So now all that you really gain from iridium plugs is longevity ie. they are capable of 100,000mls in theory but cost for cost, four or five sets of copper core standard plugs would equate to perhaps a better option as plugs can fail inexplicably and with the copper option you will have arguably tip top plugs fitted all the time. If you race or trial your car then iridium will tolerate higher temperatures and may prevent early failure so perhaps a better choice there but when I change next time, probably through plug body corrosion, I too will return to copper cored options as I really cannot feel any difference between the two options, that is except for cost!
For info 33thou in the S.I. system is 0.85 mm so a setting of 0.8 to 0.9 should be fine.
