Hi, T-rex!
As you can see from my "name", I too like the big bruisers, and would not use anything smaller than a 375 for hunting or recreation.
Here is a list of good loads for the 45-70, one of my favorite cartridges, by the way.
For all-around hunting, I would stick with the 350-gr round-nose bullet (as offered by Speer or Hornady); although the 45-70 will make a big hole, using a lighter bullet such as the 300-grainer may result in meat damage or get deflected by the smallest twig because of its poor sectional density. Instead, with the 350-grainer that won't happen, and you will still have rather moderate recoil compared to ,say the 405-grain, and when fired at game it will make a neat little entrance hole and set up nicely on the other side.
Try use:
IMR 3031 (52 gr) approx.vel: 1700 f/s
IMR 4198 (41 gr) approx.vel: 1700 f/s
Or, if you like a hotter load:
Imr 3031 (55 gr) Approx.vel: 1900 f/s
I got excellent accuracy with the first 2 loads, and they will perform beautifully on game.
If you really want a wonderful big-gane load that you can even take to Africa (my dream...) try this:
Bullet: Barnes 400gr RNSP
Powder: IMR 3031 (48 gr)
Appr. vel: 1800 f/s
This load will kick, but it will be absolutely awesome on game up to Grizzly or Moose. And remember, the heavire the bullet, the less the deflection and the meat damage.
If you just want to enjoy yourself with your new rifle while not turning your shoulder an iridescent magenta-purplish color, try this:
300 gr. bullet (Hornady)
Imr 3031 (42 grains)
Velocity: 1350 f/s
If you have to buy just 1 powder for the 40-70 go with the IMR 3031, and you won't be disappointed.
Let me know how it works!!!