I've got a Marlin 45-70. Great gun but I don't really lean on it, as far as ammo pressure goes. I typically run closer to trapdoor loads, or a little more.
If I were going to run a Marlin hot, I'd prefer the 444, as it has more steel around the chamber.
The 45-70s do let go now and then if one gets too adventurous.
The weak spot in the 336 action is not the chamber walls. The weakness is bolt thrust increasing headspace and stretching the action around the ejection port window. The lack of an ejection port on the Win 1886, along with the big locking bars, is the reason it can be loaded about 8-10KPSI hotter.
