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Hi Marco, nice to know the "real" name of the person behind the nickname!
Measuring the plate voltage at the damper is *generally* safe if the flyback is of the autotransformer variety (almost all of the italian ones, btw), but the one in your westinghouse and a very similar one employed on some early Radiomarelli sets is a true transformer with separate primary and secondary windings. When you want to measure a voltage that has past thru a flyback winding, in this case the plate voltage on the damper tube, do it with a high voltage probe, as it's almost guaranteed that there are high voltage pulses that will fry your "normal" DMM.
I know flyback transformers construction quite well, I built some from scratch and they perform surprisingly well.
About the 6AX4, yes, it's a better tube, but if you are a purist and want to retain the original 6W4 (IMHO, not recommended!), please float the filament with respect to ground, maybe adding a small separate filament winding, but be warned, it must have a good insulation! There is no reason risking the flyback transformer if (when) another 6W4's insulation fail
Cheers
Giulio
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