i have a 1893 marlin in 38-56 the base brass is 45-70 then a lot of reforming stages and annealing etc actually the round never seemed to take off its sort of a magnum 38-55 ha ha don
I was unaware the 38-56 was chambered in a 93', possibly a conversion somewhere along the line but considerable work would have to be done...I'm not even sure a 93' action could accommodate the length of a 38-56 round. Are you sure it isn't a 95' Marlin ( hopefully it is, somewhat more valuable than a 93").
Back to the OP's problem. 38-56 isn't an easy cartridge to reform from 45-70. It takes considerable force (don't use a light, inexpensive press, it might get "sprung"). The problem with 38-56 is the long neck and slight body taper that starts not to far ahead of the case web, your crunching some thicker brass than what is at the case mouth. Somehow, either with a series of specifically made forming dies or , as I did, use a bunch of other dies I had on hand for other calibers that I jury rigged to start the downsizing...use lots of lube & as many stages of downsizing that you can come up with. Just trying to downsize in one pass with a set of factory sizing dies will end up in discouraging frustrations beyond description...and a lot of wrinkled, unusable brass.
I cut my own sizing die with a 38-56 chamber reamer that I had access to at the time. I discovered early in the process that, because you are forming some heavy thick brass at the end of the sizing that just running the formed cases into the sizing die didn't make chamber-able brass...I had to take every piece of brass a little deeper into the sizing die to downsize a bit past original brass measurements and then fire form the brass in the gun once and then they worked excellent in subsequent firings. The subsequent firings were just sized in the home built die and worked good.
I will say again, it takes some brute force (especially the last smidge of downsizing). I use a heavy old cast iron Herters press built in the 50's with a snipe extension on the handle to get the job done ( with the snipe it is pretty much effortless for me but the press gets a workout).
Was all of the above worth the trouble...absolutely, My old 1895 will shoot 2" groups with those brass and I have hunted that old girl, bringing home a couple deer with it.