In bolt guns only, I've gone to 51.5gr. H4198 with a 400gr. Barnes bullet for 2,150fps. The same load runs well over 2,200fps with the various 350gr., from the Barnes "X" to the 350gr. Speer and Hornady.
: I use these same loads in my .458 2".
: In a Siamese Mausr .45/70, we worked up to my favourite (at that time) .458 2" load using H335 with 350gr. Hornady and 400gr. Barnes bullets. The charge was 68gr. for 2,150fps with 400gr. and 2,260fps with the 350's. Since 51.5gr. of H4198 gave the same velocities, I've switched to that faster burning Hodgdon "Extreme" powder. This load was safe in both the .45/70 Mauser and my .458 2" Mauser, Indeed, I even used the same load with 510Gr. Winchesters in the 458 2" M98 for 2,049fps. My .458 2" had a 3/8" leade ahead of the chamber, allowing all bullets to be seated well out. This effectively increased available powder space, as if the cae was longer than 2". No-leade chambered .45/70's should be worked up very carefully form below these loads. Normaly, one runs into case capacity problems before excessive pressures show. If one keeps powder burning rates at approximately 3031 or slower, you cannot run into pressure problems. Do not use old brass - only newly manufactured brass.
: Any data from any major companies for Marlin rifles would be a starting point for properly made Enfields barreled to .45/70. I suggest looking over Accurate Arms data.
: Work up from below and never use maximum data without previous load development. Always start from below, no matter where you get the data from.
: SOS - thanks or the links for magazines - now ordered. The side picture of the 2 actions is the best descriptor - quite vivid in the differences.