It all depends on how fast you want to drive your cast 45-70 bullets.
For fun shooting, where it is a pleasure to shoot quite a few, you will have to keep the speed down. Fast powders are used for light loads.
You could drive the bullets in the 1,300 fps range with about 16 grains of the old favourite, Unique. This is a hair slower than factory loads, but a good, light plinker.
Lyman's famous book of cast bullets shows up to 20 grains of Unique with a 500 grain gas check cast bullet, for a speed of 1600 fps.
A powder that would utilize full power in your 45-70, and useable in a 30-06, would be something like 3031, 4320 and such. It wouldn't be perfect for a 30-06 with 180 grain bullets, but could get you by. Would be pretty good with 150 grain bullets in the 30-06.