Bullet size is key in a Marlin.

My 1894 likes my Lyman 429244, but I never size them past the grease grooves, so the front driving band is as cast and that's enough to make the bullets engage the MG and it shoots like a laser.

Like you I am new to the 45-70, but with all the reading I've done at MO group and CastBoolets, and owing to the fact that 4198 is my powder of choice for my Mini 30, I'm going to start with 4198. Fewer powders is a tenant of my operation.

I've got 2 moulds on order at CastBoolets, both NOE customs. One is the 350g and the other is 425g. In both cases they are 3 cavity with two of the cavities GC and one plain base. Should make for some good loads.