There are some minor but important differences between Marlins in 45/70 and 450M. The 450 barrels and receivers feature Vee threads which essentially allow for greater material in the receiver area between the bottom of the chamber and the top of the magazine tube. Also as the belted 450 case is smaller in diameter than the rimmed 45/70 case, a smaller mag tube is used, again allowing more steel in a critical area where 45/70's sometimes fail. If you examine any of the photos where Marlin 45/70's have failed it is generally this area that lets go.
Having said that, a 45/70 loaded to around 30k pressures still provides plenty of performance and a considerable margin of safety. Besides, a 400 gr pill at 1800-1900 fps would take care of most animals roaming the earth.
