Problem with the 45/70 and smokeless is getting proper burn in that big case not exceding the guns preassure limits. That Marlin is plenty strong but you still have to watch it. Light bullet like your 300 makes it more difficult to burn completly. Look for unburned powder, if so strong crimp can help.
I shoot Single shots in ASSRA and NRA competiton lots of 45/70's on the line. Out typical rifle is much weaker than your Marlin. We use mostly black powder but when using smokeless 4759 is the most popular powder by far. It was formulated to burn properly in large old blackpowder cases.
We rarely use a bullet lighter than 400 grs, most 500's. Our loads are intended to duplicate Black Powder velocitys with less trouble and mess for pratice shooting
45/70 lever guns were never intended to use those long heavy lead bullets and generaly loaded with 300's I do shoot gallery with a Trapdoor using a 330 gr cast lead bullet but have to load with fast pistol powder to get good results. Not something recomended due to the chance of double charging, however thats what most guys use when light bullets are required.