I have an 1876 Argentine in 43 Spanish. I have just started working up loads for it with a 370 grain cast bullet. So far the loads I have found were lame, and failed to properly obdurate causing blow back.
Most loads listed are modified (read that as reduced) 45-70 loads. Most black powder cartridges listed in COW, Cartridges of the World, are all about equal to the 45-70.
I believe that the Remington Rolling Block is quite a bit stronger than the Trapdoor Springfield, and mine is in great shape, so I will use 45-70 loads as a starting spot.
I have loaded up some 45-70 Trapdoor Springfield level loads for it using Trailboss and Varget. I will go to the range tomorrow and try them out.