Well, you do realize, of course that someone did do this in .444 Marlin successfully? There was even talk of a commercial conversion business being set up by the guy who did it about 1-2 years back, IIRC. The .444 marlin and .45-70, like the .410 shotshell the gun is designed for, all have rims, so I don't think that's the problem. Dunno if there's some obstacle to doing .45-70 insted of .444 marlin - The bolt face might be too small for .45-70. Unfortunately, I think the 20 ga is gonna be too big, also. I'd probably not try to re-invent the wheel, but just go with .444 marlin, since it's known to have been done successfully, and it's nearly identical to level II .45-70 loadings for all intents and purposes.