In a pinch you could fire a 45/70 cartridge in a 45/90 chamber but it's the same situation as firing a 38 Special in a 357 mag. The risk you have is filling the front of the 45/90 chamber with debris from firing the shorter cases (especially cast loads). Fire enough of them and you may not be able to chamber a 45/90 cartridge until you clean the chamber out properly.
I'd recommend the Lyman die set as they include their M bullet expander die. If you cant source the dies you could buy cases and use 45/70 dies (suitably adjusted) to load up some ammo.
