Great ideas. Look at .405 Win. or .375 2 1/2 flanged.
I was told by another member from Canada that a No.1 MK III barrel will fit up to the large MH actions. He posted one with the barrel installed, with the SMLE Sights! Looked like a neat rig.

I converted to the .375 in a shot out .303 MH carbine. Rim and head are the same as the .303 or even the .30-40 Krag.

I have seen several MH sporter made up in .500 BP Express. these will make that loading angle easier than the .577 cases.