I have 3 45/70's and the winchester and remington loads that i have tried are really light loads.
In my Marlin I like the 350gr. Hornady JFN on top of Varget. For a lead bullet, cast my own and they are 420gr. from a mold I had made.
The Hodgdon loading book has a section on strong lever guns (Marlins) and thats the best place I have found for load data.
The most accurate load I have used is with a 400 gr. Barnes. BUT,YOU CAN ONLY PUT ONE IN THE TUBE AND ONE IN THE CHAMBER. DO NOT PUT MORE THAN ONE IN THE TUBE(magazine).