I have a 'spare' Mk III Martini action, if all goes to plan I'll make it on this lovely old calibre along the lines of what Kynoch posted - you definitely have good tastes matey!

Reason being that I already load & cast for the 500 3", so it will have to be a .510" barrel.
The 577 2 3/4" won't fit the conventional Martini Henry from memory?
DoubleD will definitely tell you one way or the other.