Milehi.
Data comes from HANDLOADER Magazine for 45-70 data, SHOOTING TIMES Magazine for .450 Marlin load data, and CARTRIDGES OF THE WORLD 4th Ed. for .458x2inch American
45-70 300 gr. 2,005 FPS
.450 Marlin 300 gr. 2,495 FPS
.458x2" 300 gr. 2,412 FPS
45-70 350 gr. 1,891 FPS
.450 Marlin 350 gr. 2,235 FPS
.458x2" 350 gr.2,209 FPS
45-70 400 gr. 1,808 FPS
.450 Marlin 400 gr. 2,021 FPS
.458x2" 405 gr. 2,110 FPS
From this, it would seem that the .450 Marlin has the advantage over the 45-70. Notice the similarity between the .450 Marlin and the .458x2" American. The 45-70 has a larger head size that the other two rounds. This would cause more back pressure on the bolt due to it's larger head size.
Ken Waters, who developed the 45-70 loads is a conservative handloader. I can personally attest to his 400 gr. load for the 45-70 as being maximum. Fact is, it was a bit too warm for my particular Marlin 45-70.
The .450 data was done up by Rick Jamison using a custom pressure barrel and Oehler PBL.
The .458X2" was worked up in a bolt action rifle.
An interesting thing was Waters also had what he called "Super group 3" loads for use only in Ruger #1's, #3's and Siamese Mausers.
Even those loads do not reach the .450 Marlin specs. Most run about 100 FPS below the Marlin.
With all that said, I think I'll just stick with my Marlin 45-70. It is powerful enough for any hunting I'll do. I can push a 500 gr. cast bullet to 1,550 FPS that will pass through an elk from stem to stern will no problem at all. If I need more power, I'll use my Ruber #1 and push that same bullet to 1,800 FPS. Either way, I'll feel my pain. Those loads hurt just a tad.
Hope this info answered your question.
Paul B.