Granadier,
Rechambering 405WCF to 400 H&H looks OK dimensionally except for rim diameter which is larger on the 405WCF (.543") than the 400H&H (.532"). Everything else looks at first glance to be slightly larger on the 400H&H than on the .405WCF. Max pressure for the 405WCF is 44,000psi versus 63,817psi for the 400H&H. I would worry about that if that same action were not available in 375H&H which is a 62,000psi max cartridge and also 300Win Mag, a 64,000psi max cartridge. Maybe the slightly over sized rim dimension is not significant.
Thanks, Singleshot