How about the .444 marlin? there is a lot more .44 bullet options out there than in .45, especially in .458 diameter... i found i never shot anything over 300 grains in my .45-70's, since there arent any big toothy critters in my area, and when i started shooting a custom .445 supermag barrel i found it would it would shoot a 300 grain bullet faster than the .45-70 did. some of the 300 grain .44s are of heavy construction so you can shoot through things like trees/mountains/small planets and should sail on through a pig without slowing down too much..
of course the .444 /.445 wont handle the big heavy bullets over 300 grains as well, so i guess it depends on how big a critter you want to go make holes in... with something that might bite back, maybe the .45-70 might have a slight edge if shooting 350 grainers or over.