The 450 Marlin is a relatively new cartridge (15 years old or better ?). The 45/70 is a much older cartridge, dating back to 1870 +/- and black powder , so there are many old 45/70 rifles that are not safe to shoot using higher pressure loads that modern 45/70 rifles, or the 450 Marlin use.
Factory 45/70 ammunition (unless otherwise stated on the box) is loaded rather mildly because of the many older and potentially unsafe rifles still around.
I don't have my reloading manuals handy right now, but if I remember correctly, a Marlin 45/70 rifle can safely handle loads up to 40,000 cup. I may be mistaken, but I believe standard factory 45/70 ammunition is around 18,000 cup.
Factory 45/70 ammunition, such as Buffalo Bore, Corbon, etc., are only safe in modern rifles, and the boxes state this clearly as well.
While I don't load or shoot a 45 Colt rifle, I do shoot and reload 45/70 and to compare a 45/70 to a 45 Colt is apples and oranges.
