Don't get too excited about stories of what the gun likes. A tremendously wide range of loads can be loaded for the 45-70. I have the Marlin with the little longer barrel than the guide gun, but otherwise the same.
I have tried about six different powders and loads so light a small child could shoot them, up to loads so heavy they made my head ache to shoot them, and I could still go much higher in power, if desired.
Every load I tried has been OK for accuracy. So what kind of a load do you want? Light? Medium? Heavy? Super heavy?
Tell us more and we can come up with some answers for you.