Another case that converts very easily to .401 is a somewhat odd one called the 414 Super Mag.
It's an extra long 41 Magnum that Elgin Gates designed. T/C and Dan Wesson chambered guns for it.,,maybe a couple of the short lived SA makers.
Starline makes the brass. Not exactly cheap,,but other than turning the rim down a little,,it's ready to go through the 41Mag or 401 dies for loading. No expanding necks or reducing bases.
Trim to Length may be needed ,but they are very close to the .401 case length IIRC.

I'd thought of using plain 41Mag brass with the rim turned down to the semi rim dia of the 401WSL,,sort of a .401 WSL Short.
It headspaced on that semi-rim anyway and the casemouth crimp holds the bullet in place,,so why wouldn't they work. Maybe they wouldn't feed?,,but that'd show up quick enough w/a couple of dummy rounds.
41Mag brass doesn't grow on trees either,,seems to be a pattern developing...
Oh well,,more time and effort into a relic of a rifle that does nothing more than shoots at targets and sticks on a dirt bank.
But it's fun.