Dawg,
Doesn't look like you are getting many takers. I have a little experience, but it is not exactly what you are looking for.
Several years ago, I guided for elk hunters in Oregon's Hell's Canyon. For a couple of years, I used a Ruger Number One 45-70.
As you know, the Number One is hell for stout and the 45-70 can be loaded almost like a short .458. I used 350-grain Hornadys with 53.0 grains of IMR-4198. Muzzle velocity was 2200 fps. Note: This load would be quite dangerous in your Marlin.
I remember dispatching several client's wounded elk with the rifle/load and it was wonderful when you have to shoot into the south end of an elk that is fleeing north (in the bottom of a brushy canyon bottom). It breaks elk down and that is what a guide wants when the hunt suddenly goes terribly awry.
During the course of these hunts, I also managed to kill my own elk with the rifle. I killed two bulls, both raghorns (a light four and a scraggley five-point). Both were shot at less than 100 yards and both dropped like a sack.
I remember one bull (the five point)went out of the scope (a 3X Leupold) so fast that I thought I'd missed him. Hey, one moment he was there with the post reticle on his shoulder. I fired and he totally disappeared.
I center-punched his shoulder and he evidently got swatted over (quite dead) like a metal silhouette. The bullet went through both shoulders, leaving very little damage and exited.
During those years, I also killed several mule deer with the 300-grain Hornady HP @ 2350 fps. Hell of an exit (always full of bone and hair). Nasty, nasty load that is OK for deer, but not elk.
Hornady lists loads for the Marlin and hopefully you can come up with something that translates to cast bullets.
Your best source, however is probably the Lyman #47 Manual. Lyman makes Springfield, Marlin and Ruger divisions for the 45-70 and has excellent coverage of cast bullets (as well as jacketed) in #47. Don't forget that Lyman also publishes a cast bullet manual.
I wrote a three-part series on ALL current loading manuals in Varmint Hunter (Issues 34, 35 and 36) and you might gather some insight from my scribblings. I made every attempt to cover the 45-70 divisions of Springfield, Marlin and Ruger are covered well.
Having said all that, I wonder what Americans would consider "Big/Dangerous Game." I've hunted Africa a few times and there is lots of it there. Here in the States, however, we are lacking in beasties that would qualify.
The bigger brown bears certainly make the list. Moose are big, but not particularly dangerous. Elk, naaaw.
Now I remember one time in Zimbabwe, when TWO rhinos ran me around and around a mopane tree for several minutes. Thank goodness I packed extra underwear. Now THAT was big and dangerous.
Best of luck with your project, my friend.
Steve