Brass for the 400 Whelen can be made a couple of different ways. One is take cylindrical '06 brass and run through a full-length sizing die for the Whelen. Try to leave the shoulder a bit long, so that you have a slight crush fit when chambering. This ensures that the brass will headspace properly. It also helps the shoulder to form nice and crisp. When you size the brass and create the shoulder, the shoulder will be a bit rounded, but will crisp up when fireformed.

The other way is start with 35 Whelen or '06 brass and expand with expander plugs, then size and fireform. It's much easier to do it the first way, but cylindrical (unsized cylinder brass based on the '06) is much easier and quicker. Less work-hardening of the brass also.