I would go with the 450 Marlin caliber in your situation since you are going for a Marlin Model 1895. According to handloading data I found a 45-70 Marlin (1895) pushes a Hornady 350 gr. RN 1900 f/s. A 450 Marlin pushes a SPR 350 gr. FNHP 2067 f/s. If you want to beat the 350 Marlin's ballistics one needs to use a 45-70 in a Rugar #1, Browning 78, Wickliffe, or Siamese Mauser bolt action to handle full pressure handloads loads.

As far as I can tell most factory loads (except Buffalo Bore Ammo) for the 45-70 Gov. are loaded to withstand (Trap Door) pressures, and do not have the advantage of full pressure loads due to safety factors, so factory Marlin 450 loads are faster. Good luck.