I think the limiting factor is the strength of the action and the short barrel. The marlins (I used to own an 1895 45/70 and it was fun to shoot!) are by no means a heavily built action. They can take 40,000-45,000 PSI maximum on a fairly regular basis. The takedown option may make the guns service life a little less with heavy loads as well because when you increase tolerances to such a level that the barrel can be screwed on and off by hand, you introduce some slop to the fitting. Every time the gun fires, the slop allows the barrel to move a bit (much like firing a gun with the butt off your shoulder.) It gets a little bit of run up before it is stopped by the threads. over time, this can increase headspace if hot loads are used often. this is the reason double rifles that are used extensively eventually need attending to by a good gunsmith because the barrels don't snap shut tight like they do when they are brand new.
All that said, I don't think you need high intesity loads for what you have listed as intended game animals. a 400-450 grain bullet at 1500fps will probably plow through a coastal brown bear stem to stern. The other thing is your barrel length.
@crs got the speeds he did because he a.) has a .45-90 and b.) it has a 26" barrel. That brought him up to about .450 NE power levels. You're not going to get that from an 19.5" barrel with a .45/70. If you want top speed, you will have to find the slowest burning powder there is data for, then work your way up to a max charge looking for signs of strain. Whatever velocity that gives you out of your gun will be what you get. I would guess, with a 450 grain bullet, that will be somewhere around 1500-1700 FPS. Hodgdon shows the max it could get in a lever action, 24" barrel and only a 400 grain bullet, was somewhere around 2000fps. Even with a modern gun like a Ruger number one, they are only getting 2,100FPS from a 400 grain bullet and a 24" barrel. The 45/70 case is just too small. You just don't want to stretch your action or have a takedown gun that rattles when you shake it side to side. I hate to rain on your parade, but I wish you the best of luck. If you find a load that is safe and gets you where you want to be, by all means, let us know!