Kevin
Check out Buffalo Arms, they'll have everything you need, and be able to answer your questions too. Everyone there, I believe, loads and shoots BP. At least, no one I've asked had to pass my question on to anyone else. Their prices are good, too.
Your basic black powder load is simple, it's a case full of powder with primer, a wad to protect the base of the bullet, and the bullet seated for your rifle.
You won't be able to get 70 grains of powder in the 45-70 case, those loads were developed with the old balloon headed cases that could hold more. You don't have to worry about pressures, just fill the case up to or just beyond where the base of the bullet will be, compress if using Goex, and seat your bullet. Leave no air space. If you compress, add more powder to full the case to the base of the bullet.
Buffalo Arms is at
www.buffaloarms.com or (208)263-6953.