A message of thanks.

Thank you 4seventy for your welcome and support and for being the firstever replier to a post of mine.

Thank you John for uploading my pictures for me and for running the best damn site on the net. Can you tell me what I did wrong as I will most likely be wanting to post pics again shortly?

Thank you Bramble for your thoughtful advice and interest which so demonstrate a willingness to help it shines through in all your posts. To comment on your reply:
I'm not old and feeble, just old. Actually, I'm not sooooo old, just 50. I am of large-ish stature at 6'3" and 220lb and, though never as fit as I'd like, I'm pretty fit. Not that any of that is any sort of predictor but hopefully I will be able to withstand the punishment of the regulation process as well as the next person. And I don't think your asking rude.
I do have a friend who is younger and much wealthier than I am who has been after me to go hunting with him in Africa (He's never been and none of his cosmo jet set friends have the interest or the experience) assuring me I'm not to worry about the expense, he'll see to it. So there is some chance this gun may see some action (though limited) in my hands. And my old friends IN Africa are beckoning me to come as well. So, maybe. Nevertheless, I'm still leaning toward the .600.
I will post a pic of the rib assuming I can figure out how to post a pic at all. I'm traveling at the moment but will be home tomorrow night. For the time being, I can tell you the crossbolt extension IS the rearmost part of the rib.
The damascus pattern is so worn off near the breech whence would come the monobloc that the metal in that area is indistinguishable from fluid steel. You're right though, I have no idea how the two widely differing steels would take the blueing. And if I do use damascus, you're right again, it would be aesthetically best for the barrels to continue uninterruptedly from breech to muzzle. With the shoe lump, is there any mechanical interlock between the shoe and the barrels? Or do the barrels merely rest in two troughs in the shoe and are thus brazed/welded, as it would appear in the Heym example of a set of .600 NE barrels in the white fitted to a shoe lump in the following pic?:

Or are flats on the bottom of the barrels just brazed/welded to the flat top of the shoe as in the leftmost diagram in the pic below from EFFEBI?

Apparently I've figured out how to attach pictures
I like the ideaa of the shoe lump for all the reasons you stated plus I wouldn't mind retaining the set of shotgun barrels if I decide not to use the original rib for the rifle barrels. I do plan on fitting a new hinge pin as well. Btw the face of the standing breech appears pitted in the pics but in actuality it is not at all.
Marrakai's Greener .577 firing pin holes measure 0.095". Mine appear in photos to be about the same size as his. I'll measure them when I get home.
When it's done I'll take you up on your offer to see you in London and prove it. That's not bar (pub) talk, that's a promise!

Thank you for your vote, 500 grains. I briefly considered the .700 for its novelty value but quickly reverted back to the .600.