or chambered to fire a 45.70?

why dont you just buy a 45/70 barrel?they a for sale all the time.

MIGHT could, but would cost far more than a 45/70 barrel

As the others have mentioned you can buy a 45/70 Contender barrel off the shelf. To your question I wonder if a 45/70 reamer would clean up the the portion of the breech threaded for the breechplug. Based on my experience with the 209x50 Encore I sorta doubt it.

Thinking further along this line.
I have some contender (old style) frames that would lend themselves nicely to an in-line black powder barrel. Seems that only the Encore line has BP barrels.
Shooting a "Great Plains" or some form of Minnie might make a nice deer rifle for the BP season.
It would seem, however, that the barrel would have to be rethreaded (or made from scratch). Wouldn't the twist on the 45/70 be too fast for a solid bullet (certainly would for a patched ball).
Just some odd thoughts as I watch this northern Michigan winter drag on in unrelenting fashion.
At least the little voices have quieted.
Mike

Michael you can get a .45 or .50 cal. black powder bbl for your Contender from Ed's Contenders. I have a carbine in 45/70 with a 16.5" bbl, it would be nice if it were longer. I too have been thinking about a muzzle loader bbl for one of mine.

btw, aren't 45 caliber bp's .451 not .458?

Not feasable as you'd have to add an extractor which sure wouldn't be cheap.