To quote my T/C load book's comment on the 45/70..."At no place within the data section is recoil not objectionable."
Sounds like fun, to me!
I remember the first time I touched off a 44Mag round out of the 14" Super barrel I got in about 1989. I had fired a fair amount of 45ACP and 357Mag so I was familiar with how to hold a pistol. I guess I was expecting a lot "more" out of the 44, so I had a death grip on it. The muzzle jump was harder to control than other guns I had fired, but the actual recoil was quite a bit less than I expected. Still, I liked it! I have found, as I added other powerful handgun barrels to the lineup, that I truly ENJOY touching off rounds that let you know you've fired a gun! When my friends talk about shooting their "9's" or even shooting my favorite defensive round, the 40S&W, I just can't get all that excited about it because they can't compare with the power and effectiveness of the Contender barrels I like so much.
With all that being said, the idea of shooting a 45/70 out of a 12" barrel is something I would do with no small amount of trepidation.
Please let us know how it kicks, ok?