Ed Matunas says the 45-70, .405 grain is fit for light big game out to about 125 yards. Trajectory makes it iffy for 3-400. Freeper elkfersupper told of long range shots with it, though. I don’t know what kind of terrain of vegetation you are dealing with.
I run into this nonsense when talking to African hunters too. I guess everyone forgets that one time a single generation of American hunters damned-near exterminated the North American bison with 45-70. We went from horizon-to-horizon oceans of moving flesh and fur to "ya don't see em no more" in less that twenty years.