Years ago when I first started loading the 50/70 I kept it as simple as possible, round balls thumb pressed into a case full of black powder. Later on I acquired bullet molds for the conicals, but I never had a load table for this round, maximum load is 70 grains of FFG black powder measured out by volume not weight. You will not get 70 grains of FFG BP into a modern case, they will hold 60-65 grains depending on the use of a drop tube, maybe as much as 68 grains. I never used wads in this load, and a lube from Dixie gun works called Black Powder Gold. With the 45/70 I sometimes down load the charge for the carbine, in this case I will add a disc of wax paper over the powder and then as many wonder wads in .45 size as needed. I only shoot one 50/70, the 1868, the 1866 spends most of it's time sleeping, even when I was making simple handloads with the round balls it was a good shooter, These days I have the dies, and such, but still keep it simple, powder bullet and lube, primer and case, that's it. 65-68 grains is your load, keep it to FFG, FFFG may let you pack more powder in the shell, but it isn't needed and will only add stress to a very old rifle. Remember that in the years after the States War troopers sent out west with their 50/70 Springfields were expected to reload for them and were issued with simple hand tools to accomplish this. No fancy presses, drop tubes, case prep tools, or high quality components, and they did just fine with what they had.
You may find that your rifle shoots best with 60 grains of powder, a bit more or less, only experimentation will tell, but always remember to use a wad of some sort if the powder will not fill the case, you do not leave an air gap with black powder. Take a case, sharpen the edge, and use it to stamp out some proper size wads from one of ET's old felt hats, soak the felt with whatever lube you would like from Dixies black powder patch lube to the homemade ideas you will find here on the forum.