Hmm! perhaps you're thinking autotransformer which will work fine. A transformer connected as you suggest will also work fine as long as the current requirements of the element and transformer capability's are respected.
My humble efforts
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Now the key here will be the leads of the 6.3volt winding. In series aiding you will get 113.7 volts output for the element, however by reversing the the lead connections it would give a BUCK situation which would further decrease the voltage to the element.
So in the AIDING situation where you'd get 113.7 VAC for the element, a 40 ohm element would dissipate 323.2 watts, that's close enough.
Willis
