My LFN with .45 nose and gas checked, LBT lube, and 296 or H110 powder. You'll probably quite wanting more velocity when your loads reach 1800 fps or so, though the gun will safely push it considerably faster. I haven't loaded for a 45, but those are the results I've obtained with a 44 Marlin, and the 45 will do it at lower pressure, though everything about the gun and cartridge will handle equal pressure.
I'm unfamiliar with twist rate in the 45, but if it is slower than the 44, you may not get good stability with a 300 gr unless you push it pretty fast. In the 44, velocity has to be at least 1600 fps to get good stability / accuracy with a 300 gr bullet.
