Don't you find 1:35 heads to be too small for 54mm? I found even the 1:32 heads are a touch small... at least when trying to to use them with Historex headgear. Maybe I'm not fleshing them out with enough hair?
So it's really a matter of searching out a head that works... or always scratch building the headgear instead of trying to make manufactured hats fit.
Could be tough when doing multifigure dioramas
Colin