There is a guy in Alaska who makes insert barrels for shotguns in a lot of calibers including, I think, 45-70. I think his email is ilred.com. Look up subcaliber adapters on your search engine and I think you'll get to him.
If you have enough money and time, you can get about anything you want made. I shudder to think what a set of complete barrels would cost but it would be fun. Winchester made a model 21 in 405 Win. some time back. It is in the museum in Cody. I lusted over that one. Maybe just coveted. Whatever.
Bill- I saw a book advertised on that subject in the last couple of days but I can't remember where. I will have to think on that.
You might try asking on http://www.doublegunshop.com/. (edited address)
I thnk this place can help you for sure: http://www.turnbullrestoration.com/
Too, you could call Brownells and speak with one of their Technicians (mostly gunsmiths) or Google David Trevallion (gunsmith)or Michael McIntosh (author).
There is a man of note in Chassell MI., I beieve his last name is Silver (?). He lives west of town on Pike Lake Rd. or Pike Creek...something with Pike in it.
I remember in the late 60's there was a gunsmith in Flint; Joe Taylor, who was doing one although I can't remember the caliber. Joe was quite the carver too and his work is collected in this area now.-Jerald MI
Browning made a Superposed that was made in 30-'06. I think it was called a Continental Express (???). Valmet has made their O/U shotguns in double and combination shotgun/rifle calibers for years
I read an article 20 years ago about a gunsmith that built a set of 30-30 barrels for a Browning SxS. It was not, obviously, a commercial success, but it was definitely doable and made a quick handling woods gun.
One of the more modern SxS shotguns may be a better choice from a strength standpoint, but I am not expert in the strength of the various models.