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The 6.3 volt CRT has a higher filament resistance. If you don't bypass part of the ballast, the current through all the tubes will be slightly reduced. So slightly I doubt any difference would be noticed. In fact it will prolong the life of the tubes to run them a little on the lean side.
Although I haven't tried a 21FAP4, I picked up to of them with the intent of using them in Predictas. I don't see any reason why they wouldn't work very well. I'm, not sure about the socket rewiring.
Sure, you could power the CRT filament separately if you want.
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