Your .451" diameter bullets will rattle down the barrel and provide no accuracy whatsoever. Marlin barrels have a tendency to run on the large side, anyway, and using a bullet that's .007" too small to start with wouldn't be worth the effort and a waste of good .45 acp bullets. You would also have to excessively crimp the bullets into place in the .45-70 case, just to keep them from falling into the case.
A better option would be to buy some bulk bullets and load them for use in the .45-70, or maybe cast bullets. I cast a couple different bullets for my .45-70's and I consider that a much better option.
Hope this helps.
Fred