The 45-70 is definitely capable, if you can get within range. There in lies the problem. I shot a young Moose about the size of a good Elk, many years ago with an old 45-70 Trapdoor Springfield. There was seven of us young GIs just out walking in the woods exploring, I was the only one carrying any thing bigger than a .22RF. This young Bull was across a slough. general consenses was 350 yards away. I set the sights at 500 yards and took the shot. I held low, right in the heart lung area. We heard the bullet strike, a solid thump. The Moose took off running. We worked our way around the slough and hit his track. This took over an hour so we felt it was time to start tracking. A little blood at first, then it gave out. We lost his track several times, so we spread out and searched till we located it again. It took four hours, to locate him and he had gone about one and a half miles from the location where we had shot him. I had hit him a little high, but it only took out one lung. In other words after going through that thick hide and a rib, it only penetrated one lung and stopped. I was shooting factory ammo, of the mild type, the only thing that old trapdoor would shoot.
My opinion is if you can get to within 200 yards the 45-70 would do the job. Any farther and I would want something a little flatter shooting, and a little faster. A suggestion, have a 30-06 barrel fitted to your BC. Then you always have backup if the ranges are too far. But the most important thing is practice, practice, practice. Few people do it enough. Know where your gun is hitting at every 50 yards, all the way out to 400 yards. I've seen people miss an easy behind the ear shot at 50/60 yards by shooting over it.
