I don't like putting scope like the 3-9's etc, on lever guns UNLESS they are the ultra compact kind. I like a small scope that I can mount low on the gun (no see thru rings). The reason is because lever guns have a very thin (compared to bolt actions), high receiver that makes them a bit more susceptible to the side to side wobbles, and having a heavy scope in high mounts just makes it worse when you're trying to hold it steady.
A scope that's 9x or 10x isn't too much power for the range of the 45-70, after all, it's shot out to 600yds and even more during competition. Range isn't an issue with it, trajectory is the issue.
I like scopes with objectives of 32mm or less for 99% of my guns. A 2-7x32 would be ok, as would the compact scopes of Burris, really the only company that makes compact scopes that have more than 9x at the high end.
What are you planning on using the gun for? IMO, I'd go with one of the scopes in the previous posts above, or maybe a straight 4x scope like a Nikon ProStaff 4x32, the Sightron 4x32, or the Leupold 4x33. All of them are compact, won't affect the balance of the rifle very much, and have good enough glass for any shooting you'll realistically do with the 45-70. More importantly is that all have at least 4" of eye relief.