Ed - some time ago, Les Bauska used to make up overly-large Ruger-style #1 Actions chambered for the .600 Nitro. A note to his son might shed some light on this for action size, rim size allowance, etc, and if the outfit who cast the actions for Les is still in operation. A huge #1 Action would make a fine modern 8 bore me thinks - kinda like so many English guns, but hammerless, of course.
: I'd really like to take you up on your offer of the reamer and cases, but alas, my shoulder will no longer take such punishment. Woe is me. Some day, we have to meet.