According to the Michael Petrov article cited above, the original shoulder diameter for the .400 Whelen was .458", which is what I was comparing my fired cases with. Cartridges of the World gives .462" as the correct dimension of shoulder diameter(no citation).

The Handloader's Manual of Cartridge Conversions lists the cartridge as "Rimless straight"(!)with no shoulder diameter given, referencing Phil Sharpe's Complete Guide to Handloading (p.398). Sharpe quotes Colonel Whelen as being "lukewarm" about the cartridge and describing it as difficult to handload for because of headspace problems caused by the "faint indication of a neck"(!!).