My original load for the 45-70 (which was recommended to me 25 yrs ago) was 19gr of 4759. It still works, and is very light on recoil. I have also used Unique, but recently we have had the best luck with Accurate Arms 5744 (23-25 gr) and 4759 (22-23 gr) with a toilet paper wad.  After LOTs of testing, my neighbor Tom and I believe that a toilet paper wad does improve accuracy with both 4759 and 5744. I use 1/4 sheet, Tom uses 1/2 sheet.  I know the subject of wads is controversial, but I havent heard that anybody ever had a problem with TP, and so far we havent. With a 500 gr bullet, 22 gr of 4759 has always worked well for me, and 23-25 gr of 5744 for the 420 gr bullets.  I dont know if I have tested 19 gr of 4759 with the TP wad, I started going with 23 gr of 4759 a few years ago because I was getting better results at 200 yds with it. With the wad , the 19 gr might be good. 

As for alloy, I have been using 50/50 WW and lead pipe. Anything softer causes the bullets to jam in the chamber on unburnt kenels of powder, which you will get with both 4759 and 5744. Smokeless in the 45-70 is great, I have loaded and fired well over 20,000 of them. However, once you get beyond the black powder velocities, recoil can be a real problem. Recoil isnt a problem with any of the loads I am using. Good Luck!  Joe S