There is a guy who rebarrels rolling blocks on gunbroker. He auctions the service for about $150 if you send him the action and a barrel blank. Somewhere I have a link to some place that had 45-70 unchambered blanks. PM me if you need me to dig it out.
That action should be fine for any of the black powder cartridges, rifle or pistol. Like Frank said, as long as you run the 45-70 at apropriate load levels it will be fine. Those rifles were built in 43Spanish, 45-70 and 50-90. The later ones were stronger, but as long as it's not cracked the early actions are fine for the cartridges they are made for.