There are typically three levels for loading the 45/70. The first being the "Trapdoor" loads, which hold the pressure at the original level when the round was introduced. Second is the "Lever-action" which may go to 40K CUP, and third is the level for Siamese bolt rifles and others with the same action strength. These can go 50K CUP.
Laser cast is probably keeping the pressure very low in deference to the Trapdoor class rifles. Also, is the bullet hard-cast and gas-checked? Leading could be an issue.
Hodgdon shows 55 grains 3031 with a 400 grain jacketed bullet with 37.3K CUP and 1971 fps as a maximum load. The Marlin is good to 40K. Substituting a cast bullet for a jacketed one would likely lower the pressure.
Their trapdoor section shows a starting load of 45.5 grains of 3031 for 17.3K CUP and almost 1600 fps, so your load would be pretty sedate.
