I would not count out the 45-70 with upper trapdoor levels for a 100 yards gun. You don't have to use Garret, buffalo bore, cor-bon to get the velocity to do the job at 100 yards, a good cast factory round from Federal will work. Personally I would highly consider reloading. That is the best way to tailor your loads to your needs and have the ability to use the bullet you want.
You will still be shooting a 405 gr. bullet with a 45-70 at upper trapdoor levels faster than you can shoot a 300 gr. 44 Mag. If you loaded up some 300 gr. bullets in the 45-70 at trapdoor levels, you would not be able to touch them with any 44 Mag load with a 300 gr. bullet.