As far as I know, no one manufactures a handloading carbide sizer die for the .45-70.
Yes, the case is straight-walled, but it has a slight taper like the 9mm luger and the .30 US M1 carbine. Those other two have carbide dies because enough sales demand warrant their production.
A custome-made dies is certainly possible, of couse. Are you willing to pay the several hundreds of dollars to have one made?