I picked up a very slightly used (<20 rounds) 1895 Winchester in .405 - I tell you what, when I bagged a decent sized hog with it, all that ran through my mind at the time was remember back to when I was a kid watching a boxing match hearing Howard Cosell screaming "Down goes Frazier, Down goes Frazier".
No matter what cartridge you choose, my advice is to opt for the biggest diameter projectile you can stand and go from there. While I got my rifle for a song, it would be very, very difficult to walk away from a Guide Gun in 45-70. Big cartridge, great platform, BIG hole - sounds like the perfect hog gun to me!