Are the 450 Marlin and the 45-70 cartridges interchangeable.....in a word no. The 450 has no rim on the case, so the barrel will not be machined to accept one. The action would not close because the 45-70's rim would interfere. If a 45-70 was fired in the 450 chamber, the case would buldge to fill the space in the chamber for the extra wide belt on the 450 case.
I think there's a lot of experimentation to be done with the 450 Marlin cartridge. Enjoy yourself!!
Lobo Lohr