GarryD007,
For the 450/400 2 3/8", you might be able to use a procedure similar to the one skeetx described for the 500/450; but use 41 mag sizing die.The Lyman"M" mentioned by skeetx has a 1/2"-20 thread (depending on make- the 41mag expander may also have this thread), so an expander/belling stem can be easily made from a hardware store 1/2"-20 bolt.Iron the "bell" out with the 41mag seating die.I would guess this would work, but I never loaded 450/400-2 3/8", so I'm not sure.
Mike