I'm a little confused
Are you talking black powder or smokeless. I dont know if your going to find any smokless data, I went through 3 of my loading books and there is nothing on the 50-70.
Some books have loads for older 45-70 loads and if you reduce that by 10% or better you should be ok, but really if your shooting older guns made for black powder you really shouldnt shoot smokless.
If your shooting black powder, and since you havent said one way or the other the 50 is diamater and the 70 is the ammount of black powder. If your looking to reduce the load you have to be cairful about casses that are not fully loaded with powder, the primer can flash across the top of the powder and cause very erratic pressure changes and can be devestating to the gun and shooter.
Also any one that shairs load data must add the disclamer that was put out on one of the threads from a staff member. This is required. and a good idea.