If you want to go with adifferent case length, go with the 45-90. The brass is easy to get and it is a great round. I have one in a C Sharps and it is great. I do shoot mostly smokeless powder with 500 grain + bullets. I shoot paper patch mostly. If you use slow burning rifle powders, i.e. R22 IMR 4831 AA8700, you will have lower velocities in the 1300fps range and will probably get very good accuracy. I use Hodgdon Varget, AA2495, & H4895 for heavy hunting loads. These will give good velocity,1800 fps with 500 grainers in my 30" barrel with low chamber pressures. Recoil would be really stout in a #3. They are a real treat in my 10.5 pound Sharps. They do kill like crazy. If you are killing bowling pins or bowling balls, you will get one shot kills with the low velocity loads. By backing your 45-70 dies off the shell holder in the press, you could load the 45-90s just fine. It is great caliber.
Good shooting,
Dave