Daryl, Qual was doing a CYA in case anyone went down to a 30 caliber or below. Then I would agree with them BUT since I have a 400 Whelen AND have made cases from Norma/ Qual basics as well as Rem 35 Whelens, 9.3x62 (both too short) AND have shot probably 500 rounds thru my 400 (95% cast bullets that mike .412), you might consider I have quite a lot of experience with the cartridge.

All things being equal, the smaller the hole you end up with the more critical the neck thickness.
My 22-06 Sedgley has a very tight neck and will only work properly (new bullet will drop in fired empty case) with WW 25-06 brass (no nickel plated) cases.

The key is having the correct G&H Col. Whelen spec chamber (as covered in Pertov's chapter on the cartridge in WILDCATS bt Zeglin).

Once there, just follow my instructions in this thread and you'll have a fine 100% dependable rig.

It is a simple cartridge to load, is very accurate and has as much thump as a 450/400.