The original .30/30 case was a target cartridge in either .32/40 of .38/55. There was a .38/56 and as a kid I wondered the difference. It is the .45/70 case necked down. No shoulder. .40/65 too. Since your Marlin is available in .45/70, no reason to not think in terms of .38/56 WCF. For more steam, there is a .38/56 improved. Mr. Richard Corbin built one on an 1886 Winchester, I think. [rceco.com]. Many .45/70 cases around. (Many brands if precise accuracy is important to you).
Then there are the triad cartridges. 'Smith designed them to use factory dies and .444 shell holder to reload. 7mm. .30. & .35. No reason you couldn't have a .375 if you asked. I would guess google or others could bring this up for you.
eabco.com has worked with the .219 Don Wasp a bit. that is the .30/30 necked down. Makes cases. Might ask there too. luck.