The rifle forearm for the G2 has a straight channel for the 23-inch factory bull barrels. If your .45-70 barrel has no taper, it will work. There are Contender Carbine forearms out there. They are for the pre-G2 21" carbine barrels. They were also used for the 16 1/4" barrels. I have one of those. They are shorter with a slightly tapered channel.
I have a Fox River tapered barrel. But it does fine with the regular G2 forearm. Since it is longer than the carbine forearm, I use it all the time.