It seems that a .400 Whelen should have some headspace issues given the small shoulder...however with a properly cut chamber it should be no worse than a .45 ACP or a .458 Winchester or a .41 or .44 AMP.

If the headspace problems exist with a properly cut .400 Whelen chamber, that would explain why it is extremely uncommon.

Personally I'd ignore the short cartridges and stick with a fairly common .375 H&H and just load it down a bit.

Richard.