This is bullet weight that is quite out of the norm for the .45-70, unless you are shooting long range blackpowder competitions, or perhaps blackpowder silhouette.
I suspect that the throat of this handi-rifle might be short as well.
Perhaps a load of blackpowder, that allows for the bullet to be seated to match the throat, with about an eighth inch of compression of the powder once seated? That could range from 55-65 grain of BP. It might not be worth your while trying to do a load for 20 bullets that are that odd ball for your rifle.