.375x2 1/2" would probably work ok, but I have a .410 barrel so I want to use that.
I already have one in .303 and would like a bigger brother for it.

The 450-400 3" was rated at 16 tons ( 35,840 lb ) /sq ", so I'm not worried about the pressure or the strength of the action, I intend to shoot cast bullets from it anyway.

Looks like I'll have to make a dummy barrel shank and see if the round will feed easily.