Yes some of the "45-70" bullets are really intended for the 458 win mag. You have to pick your bullet carefully. I believe the 350 Hornady is one of those made for the win mag. The Rem corelokt bullets are softer and designed for slower speeds. The 300 gr Rem Corelokt bullet would probably be all you would need if you can get them to group for you. I have shot the 405 Rem's before, and they are VERY soft.
The Lyman reloading manual has a section in it for TC Contender loads, and they do list 45-70 as one of those chamberings. You are also loading to "Springfield Trapdoor" levels, if you are using another manual for reference. I would start with a modest level of IMR 3031 or RL-7 and don't push it past "Trapdoor" levels.
I'm working with cast bullets myself, so I can't really help you with a jacketed load. Right now I'm playing with a home cast 350 gr Lyman, and a "store bought" 405 gr hardcast.
I still do most of my shooting for "fun", and shoot heavy blackpowder loads out of it. But, I have become interested in trying to push some accuracy out of it, and am starting to work up loads "for real" use.
Whitetails are NOT hard to kill. Pretty much whatever you shoot them with will work. We're still talking a .458 diameter hole in a deer here. As long as you "steer the hole" through the vitals, you're gonna kill a deer.