I know about the old Colt Lightning and owned a Savage 170 for a time. Those along with your .12 gauge pump are tube magazine fed. The oldest Remington pumps the models 14 and 141 are in this same catagory. I seriously doubt any of these designs could take hot loaded .45-70 at 45,000 psi. None of those have been chambered for cartridges approaching such pressures as the hot rodded .45-70. And to add, 1880's big bores were low pressure black powder cartridges that couldn't approach those pressures at the time. Makes the .45-70 even more special doesn't it? Love the cartridge in my Marlin lever action.
With that in mind, I guess I sorta fixed my mind on the 7600 Remington pump rifles and their detachable magazine. That big old rimmed .45-70 might be kinda hard to cram in that "clip" of the 7600 and make it feed reliably. The .45-70 would need to be stacked with rims positioned just so, one ahead of the other directly over the top of each other. It would also need straight line feeding, and, that magazine doesn't seem a likely candidate for such a conversion. That's what hit my mind when I read the post.