Same here, Mart - neck or partial FL sizing only - for everything I load for. I use a set of .375 Styer (larger body diameter) dies for my .375/06IMP loading - only sizing most of the neck. Prior to getting the Styer dies, I used a neck bored .330 Win mag. die for this. Hornady Styer set is really nice, in comparison to my altered dies.

A Hornady .375 Styer die, bored out would make a neck sizer for your .400 Whelen IMP. Normal Ackley IMP shoulders are .454". My .375, made with non-standard Ackley reamer, has a .460" shoulder. I neck .30/06 or .35 Whelen brass straight, then neck size to set a shoulder for fireforming. Even though they are set for a crush fit, I seat the FF load's bullet out to hit the lands in the throat for perfectly formed cases.