I'm of the opinion that you really don't need to hotrod the 45/70 if all you're going after is deer. A .45 cal, 300 to 405 grainslug travelling at 1300 to 1700 fps is going to leave a big hole and isn't going to stop for much. This is, of course assuming that you're using the proper bullet construction and are within the rifle's range limits. If you think you're going to be shooting past 100 yards a lot, the .45/70 might not be the best choice. However, if you're willing to get close, there isn't much it won't kill, even with factory ammo.

I have an 1895 with a 22" inch barrel. I prefer that length because my eyes happen to line up a peep sight better when I have a longer sight plane. That's just me and there are plenty of people who love their guide guns.