Leading is influenced by velocity and bullet hardness, and also by bullet diameter, bore condition, and even the powder you are using.
You're likely not going to get more than 1500-1600 fps out of those bullets in a 45-70. It might be physically possible to load them faster but you might find yourself reaching .458 magnum pressure levels that probably aren't within common sense guidelines in a 45-70 lever gun. Your bullets don't need to be super hard to make 1600 fps with no leading.
I regularly push flat based cast bullets to 1600 fps without leading. A cast bullet should be .001" to .002" larger than groove diameter. Undersize bullets are much more likely to cause leading than oversize bullets. Bullet "fit" is the single most important factor when shooting cast bullets, and the reason experienced cast bullet shooters keep repeating the mantra: "slug your bore".
In rifle bullets I like 18-20 BHN lead hardness. In my experience, bullets made from a super hard alloy tend to cause more leading problems. I'll shoot flat base up to 1600-1700 fps and use gas check bullets for anything over that. But if the bullet fits well and the rifle bore is in good shape, then it is entirely possible to shoot flat base cast bullets at over 2000 fps without leading. Our member "Why Not?" has done so recently with flat base 9.3mm cast bullets in the 2200 fps range.
For a rough reference:
8 BHN: pure lead
10 BHN: you can easily dig a groove with your thumbnail
15 BHN: can make a light groove with your thumbnail but it takes more effort
20 BHN: hard to make a small scratch mark with your thumbnail but not impossible
28 BHN: probably can't make a scratch mark with your thumbnail unless you have hands like a grizzly bear
Obviously this is subjective, but might give you some clue about hardness. A hardness tester is the only way to reliably say what the hardness really is.
The propellant makes a difference in the velocity you can obtain before leading starts. For instance I was having issues with leading in a 45-70 using 425 gr bullets with IMR4198. I got some advice on Cast Boolits Forum to switch to a slower powder. I tried IMR4895 and had no more leading using the same bullet at the same velocity. Since then I've noted this same tendency in other cartridges. Slower powders quite often work better than fast powders when the velocity starts to get up past 1300-1400 fps. It's a fairly well known phenomenon amongst cast bullet shooters.
If I were the OP, I would try those bullets in the 1500 fps range and see what they do. If you are using these bullets purely for "bear defense" (aren't we all??) then a bit of leading from a few shots probably are not a big concern anyway, except for the effort needed to clean the bore afterwards.
