I noticed your post in the gun owners group, I also have a 577-450 Martini Henry.
I have an insert that let me fire .45-70 in it. Basical it's a stainless steel 577-450 Martini Henry with a .45-70 shaped hole machined out of the center of it. The bullet diameter is the same and the 577-450 Martini Henry is physical larger and more powerful so you are adding material in the breech to fire a less powerful round. It might be something to look into. I would like to be able to give you a link to find one but I got it from the previous owner.
There are two versions of this video. This is the one I remember. The other featured a woman tossing her boyfriend/husband's junk out a window and was shot in black & white.
Matti