Just about any centerfire rifle from 270 on up will be ok for bison. But if you're hankering to use a .45-70 for bison (which would be my choice for nostalgia reasons, actually .45-90) Marlin makes a nice reliable lever gun. And they also make a 'light weight' version in the Guide Gun. If your wallet can stand it, you can also get an old Winchester 1886 in that same chambering or a Sharps 1874.
But remember as far as weight and recoil - the lighter the rifle the more felt recoil your wife will feel.