I don't know, but I suspect a shot load might not perform that well. Rifling tends to spin the pattern quite a bit wider than a cylinder bored smooth barrel would. If you were going to go with a shot load, you could get some of those .45 Speer shot capsules. You could probably adapt some of the data they provide to make them work with the 45-70. Plus, with a normal shot load,without the capsule, you would probably get some massive barrel leading. Speer intends these capsules to work in 45 handguns. You fill them with shot, and seat them like a bullet. I guess the capsule shatters when it hits the rifling, but still stays between the shot and bore, keeping the bore clean.
A better option might be a gallery type load using an appropriately sized round ball. If you play with the powder charge sometimes you can get a light roundball to strike point-of-aim at a short distance, and not have to ajust your sights. If you see a bird on the ground at 12 yards you could pop it in the head with a roundball.