Congratulations on choosing the .45/70. The cartridge is more versatile than many give it credit for. A 350 gr .458 compares favorably with a 170 gr .308, so if matching the trajectory of a .30/30 matters, it can be accomplished with the lighter bullet. But where the .45/70 shines when its loaded with heavy bullets. A heavy flat nosed .458 bullet kills heavy game with authority, and kills light game without blowing 20 pounds of meat to mush, the way a high velocity cartridge, loaded with light jacketed bullets does. Shooting cast bullets poses no disadvantage in a low velocity cartridge like the .45/70, and while shooting cast isn't free, it does mitigate the ammunition cost, compared to jacketed bullets, even though each bullet weighs about an ounce. Your cost is further reduced if you cast your own bullets.
Trajectory is a constant. Because its predictable, you can easily adjust for trajectory across reasonable ranges, even with a .45/70. Stick with one good load; zero the rifle for 150 yards, learn the trajectory, and you'll find that shooting beyond 200 yards isn't that tough, provided the sights on your rifle are suitable. I'd quickly replace the factory sights with a ghost ring and post; the wide flat top of the post provides an index for elevation, similar to the horizontal wire of a cross hair. That index of elevation is absent with a bead front sight. You might prefer to mount a low power scope on the rifle, and if you chose a reticle with holdoff points, hitting at longer ranges becomes more viable. A mildot reticle for instance, equates to 7" between dots, or hash marks, at 200 yards, and 10.5" at 300. A good quality, low power scope, is arguably as quick to use as irons hen the range is short and the action is fast. The scope's disadvantage occurs in rain, fog, or heavy snow where lenses become coated with moisture, rendering the optic unusable. QD rings provide you with the option of choosing the sight that is most appropriate to the conditions you face.