Actual black powder works much better in the older guns than does the substitutes. It has to be compressed about 1/8" or more when seating the bullet. You will get much better results by using a long drop tube of 12-18" when charging your cases. Use dippers or a black powder specific measure.

Bullet lube must be soft and SPG seems to work best. Air in the cartridge can well lead to damage to the gun.

Modern brass has less capacity than the old balloon head cases. You will be unable to get 70 grains in a 50-70 case. Although black powder ignites easier, magnum primers seem to give the best results. Because so much black powder is unburnt after the shot, you don't have to worry about charge differences of a grain or two when loading.

Good luck. I suggest you spend some time on the 'net, particularly castboolits.com, before starting this endeavor.