Same as Daryl & Steve, back out the FL size die so it only sizes the neck..

The quick and easy way to neck size would be a .41 Mag die if there is no better available option. Then if necessary alter the expander plug to specific diameter.

Still and all I would bet that any .450/400 3 1/4 (or 3”) die would size the neck enough to work with a .408” bullet.
Alter the expander plug/ball to suit.
- Mike