Exausa - your chamber is closer to the standard IMP chamber, which is normally .454" My barrel maker here made me a .375/06IMP with a shoudler of .460" and base of .470". .005" taper per side garantees perfect extraction and excellent pressure containment while giving an even larger shoudler for headspace.

The original .400 Whelen had a smaller shoulder - slightly larger, I think, than the normal .441" shoulder of the '06, but not by much. This was why so many hd difficulty, either real or imagined about it's headspace.