The 45-70 is easily reamed out to a 45-90 or 45-110.
With modern powders, you do not need to go further.
In my Miroku 1886 45-90, I get 2150 fps with the 450 grain Kodiak and that kills Cape buffalo well.

My friend in Australia kills their water buff with a 300 grain .405 WCF bullet, so with proper 45-70 ammo, you do not need to extend the chamber unless you really want to see what you can get out of the rifle.
If you do extend to 45-90, please let us know how it goes.