I understand your concern with the large porkers. The beauty with the 45-70 or similar large caliber cartridges is that you don't need all the much velocity.
In the kinetic equation for energy the velocity is the largest part and in smaller cal it is needed to generate some decent ftlb's. When you start hurling 300 to 500 gr. of bullet, the mass takes on a significant role. Now that I have run out of fancy terms, here is what it boils down to:
Bullets in the 350 to 400+ gr weight do not need to fly very fast to do to a number on a boar or anything else.
If you send a 400 grainer at 1400 to 1600 fps MV to one of those walking porkchops it will kill it if you do your part and put it in the right spot.
If those bullets were to be hard cast they would probably penetrate and kill two hogs side by side without a problem.
BTW be careful if you use that 350 gr Speer bullet as mentioned above. I loaded some for the 450 Marlin and found out that they chamber ok even from the magazine but when you try to unload a unfired round you run into trouble big time. The round will not clear the ejection port. You can't remove the bolt without getting at the bolt stop which is hidden by the round. Short of tapping the bullet back into the case while the round is chambered
you have two other choices.
Firing off the round is the simplest way if you are in a place you can do this. I wasn't in the position to do that so I had to disengage the extractor from the cartridge.