The .400 Whelen was a problematic cartridge from the beginning.  It is a rimless case with no shoulder.  Rimless cases typically headspace on the shoulder and there is none.   The rim on this cartridge is only .003 larger diameter than the case head.  In those early days of reloading the cartridge developed a bad reputation quickly. It was never adopted as a standard by a firearms manufacturer.  I am surprised that you were able to find new brass with that head stamp.

I've got a friend just got a 375, I would love to get brass for it so I can load for him without cutting down 38-55.

400 whelen was only problematic because of trying to make it headspace on a non-existent shoulder.  I did a cartridge similar, as did Ken Waters.  We later compared notes and had an interesting and parallel process/reason for making it. 

Mine is the .429 express, which is a 30-06 case blown out straight.  It headspaces on the mouth.  Kens is the 444 marlin rimless, also 30-06 blown out straight and headspacing on the mouth.  Difference is length of the case, mine is longer, his is identical to 444 marlin.  We both use 444 marlin dies to size the cases and load.  

Mine is 2.333 inches long, and can push a 300 grain bullet at upwards of 2600 fps.  It headspaces flawlessly, and case life is 10 or more loadings.  

I'll check out starlines 30-06 basic.  If it's long enough, it'd save my scarce 30-06 basic (2.65" long) Jamison I had Marc make specially for me for wildcat cartridge forming.

P.S.  I went and looked at Starlines site, at 75 cents apiece and 2.5 inches long, the basic brass looks perfect.  Thanks for the heads up here on it. 

I'd be interested in both the 375 and the 400 whelen brass OP has, depending on price.

The 400 Whelen  brass is Quality Cartridge and the 375 is Starline.

Hello, Is the 400W marked as so? I have a buddy building one. He would love to have them. If selling? How many and what's the price? Plumloco

The late Michael Petrov used the 400 Whelen and wrote a few articles on it. He concluded most 400s had been made with an incorrect chamber reamer, one which left too small a shoulder. There should be .018 for the shoulder, which is about what the 45 Auto has to headspace on. With correctly formed brass, he had no trouble with it.

Further down in my notes I see another fellow found an old American Rifleman article by Whelen on the subject. It gives case dimensions of .455 at the shoulder and .431 at the base of the neck. 

That neck is smaller than other sources. Perhaps Whelen meant to turn down the neck a bit to give a little extra shoulder?

Petrov's numbers had come from a chamber cast of an early 400 rifle. That rifle worked perfectly with cartridges made with Whelen's own dies, which he'd been able to borrow.

It's a wildcat. Pacific Tool will make it whatever shape you want. Whelen designed it for 405 WCF bullets. Using the heavy bullets for the 450/400 NE, an -06 case should get one performance close to that of the elephant rifle.