Critch,
Pay attention to Paul and the #1 issues, he is a #1 kinda guy. I am exactly like Paul when it comes to the Marlin, I own two, it just aint that strong and popping a lever is getting danger close for no legitimate reason.
I have a few #1s but none in the big bore range. On the other hand, I do own two Siamese Mauser based 45-70 guns. They are in my way of thinking, superior to just about every other 45-70 chambered configuration when it comes to pressure. The Siamese is a late 98 Mauser built by Arisaka for the Siamese government, chambered for a rimmed cartridge. I also own some real shoulder beaters, like a 458 Lott and a 20-inch carbine in 338 WM.
Probably the best thing to talk about in the 45-70 in the Mauser configuration is the fact that you can not get enough powder in it to flatten the primer until you go to a pistol powder like H-4227. Yeh, well, after several hundred test rounds, I got tired of firing a round and it dropping out of the chamber with a round primer so I loaded up a 525gr RCBS cast until I got a flat primer so I satisfied myself that it could be done.
Mauser Only:
With a 300gr Rem HP you can get 2,600 to 2,700 using R-7 at about 1 inch at 100
With a 300 Rem HP you can get 2,500 fps using 2015BR at .75 inch at 100
Here in the SE I have not seen a need for the bigger bullets in the 45-70. A 300 at 2,600 to 2,700 fps is about all a 150-pound whitetail needs for a clean kill.
The Mauser is my illusion gun... causes a deer to just disappear.
Best,
Ed