In general, the parallax is set at much closer range in keeping with the range of a shotgun shooting slugs. Generally around a 75 yard setting, as opposed to a hi-power rifle scope set at 150 yards.
And they all seem to have much thicker reticules, or some trick reticule that fits a turkeys head instead of a duplex cross-hair.
Still, for a 45-70 and the ranges you would likely be using it at, I can think of no reason a shotgun scope wouldn't do just fine. The thicker reticule might even be an advantage in deep dark woods.
rc