Give me a break. We aren't talking grizzlies are we? Plenty of folks hunt black bears with BP guns.

The only question I'd have is does your rifle stabilize that slug well enough? It's pretty long at .45 cal. and 460 grains. As long as it shoots well, more power to ya. On the elk question, It may be "too much" bullet if you have to shoot a ways, depending on the powder charge you can put behind that bullet. Just find out how much the bullet or your shoulder can stand, and see if you get acceptable trajectory for the distance you expect to shoot.