My first suggestion is to try whatever you shoot in the Marlin. I've had a 10" BFR in 45/70 for about a year now and only shot handloads in it so far. I haven't found anything that didn't shoot well so far.
My favorite short range load is a Cast Performance 440gr WLNGC over 45 gr of IMR 4198. This is a pretty stout load, but it shoots well. I have the Millet white outlined rear sight, and the Hi-Viz High fiberoptic front sight on my gun and with this load I can zero it at 50yds and have have a 100yd zero with my lighter load of a 300gr Hornady over 57gr of IMR 4198.
The best allround load I have is a 400gr Speer over 44gr of IMR 4198. It's a little lighter than the other two loads, but it's easy on the hands and the Speer bullet performs great with this load.
