Dale,
since getting my 1895GS I have learned something about the Marlin 450. For one thing the rifle is stronger than the 45-70, in so far as the threads on the receiver are a different design than the 45-70. ...
The newer design is stronger.
...
If you reload, I'd actually suggest the 450 over the 45-70, because all bullets, in the Marlin, are cross compatible, and should you desire to punish yourself, you can most likely load the 450 to a slightly higher velocity.
You cannot, however, get a 450 Marlin guide gun in stainless, should that be a desire. No one seems to know why either.
The 450 thread design may result in a stronger action, and probably does. I do believe the threads provide better mating between the receiver and the barrel and improve accuracy as a result.
The question that needs to be asked is “Is it required?” Or is it like having two very similar cars, one that does 135mph and one that does 145mph, when both are used as family cars and never go over a 75mph speed limit? Its been my contention from day one that a primary reason for Marlin using a different thread design on the 450 was to prevent the possibility of mismatching parts during assembly (a 450 receiver with a 45-70 barrel or vice versa).
As to reloading, the data suggests the 45-70 can reach higher velocities than the 450 when loaded to similar pressures. Case capacity wins this battle and the .45-70 has the larger case.
Why Marlin didn’t make a 450 Guide Gun in stainless is beyond me. Seems like they would have sold quite a few and helped the 450 cartridge along in the process.
