45-70
Personally I like RL7 for my 45-70's. The cost of jacketed bullets in the .458 caliber is ridiculous, and the jacketed bullet has no advantage over a good cast bullet in the 45-70 cartridge. Personally I cast my own 405 grain bullet and back it with 40.0 grains of RL7 which will yield about 1600 fps through your Marlin.
This is a well balanced load. Heavy bullet, a real killer. But not a thumper on your shoulder. The perfect mix for a 45-70.
I can cast and reload about 500 or so rounds for $100.
Take your time, buy some inexpensive(to start) casting equipment, and shoot that rifle. Once you get started you will never look back.
All you really need to start is a cheap pot to use on a cook stove, an inexpensive mold, and a dipper. You can pan lube your bullets, and you can pick up some lead some where. Tom.