The .375 x 2 1/2 works quite well in the Martini...I have one
CIP says the 450/400 N.E. 3" is a 2800 bar cartridge, that's over 40,000 psi, not something I would feed as a steady diet to 100 year+ rifle, even a Martini.
The 450/400 is a BN so it might feed, but it's not a cartridge I would shot in a Martini.