Matt,
I don't have my manuals available at the moment, so I can't suggest a load, but I do use H4895 in my .458 Win. This powder works very well in a straight walled case, and gives good velocities.
There are enough 45-70 shooters on this forum, so I'm certain that someone will speak up with an appropriate load for you to try. Good luck with your 45-70, and approach your loads gradually.