If you are going to load the 45-120 to the same velocity as the 45-70, I really dont't see any benefit.
Anything you gain in lower pressure due to the greater case capacity will be lost in the dramas of trying to fill all that extra space.
I wouldn't rechamber one of these rifles to anything.
Sounds like you want a 45-70. These rifles come in 45-70. Pressure is already very low at 28,000psi.
Why would you want to complicate things with a rechamber?