The Brit name for the ctg. cited above is the .375 Flanged Express 2 1/2". A good choice if you want to use heavy bullets. Only caveat is to make sure the .375 reamer would clean up the back of the .30-40 chamber; it is a very tapered cartridge and I'm not sure if it is as wide as or wider than the Krag ctg. just in front of the base.

An advantage of the .405 which I'm sure you are aware of is that there is brass and ammo available. Not sure if a rebore to .405 would leave sufficient "meat" in a Krag barrel; might need a rebarrel with a slightly heavier profile blank.