What CalvinMD says is spot on. Get 45-70 brass, Full length size in a 45-60 die, then trim to 1.89" and chamfer the mouth.
This cartridge is not in the same league as the 45-70. The twist is generally slow enough that 300 grainers are about the heaviest bullets that will stabilize.
A published smokeless load from the Handloaders Manual of Cartridge Conversions shows a 300 grain cast bullet over 25.0 grains of 4198 powder for a velocity of 1450 fps.
Cartridges of the World 6th Edition shows a 210 gr bullet with 21.0 grains of 4198 for a velocity of 1520 fps and energy of 1083 ft lbs. The bullet being used in this load is cast in a Lyman 403168 mold. Sighted in at 200 yards the mid-range trajectory is 11.8" above the line of sight.
The same book also shows loads for the 45-60 Marlin, which for all practical purposes is identical and fully interchangeable. This give two more useful loads.
A 260 grainer (Lyman 403170) with 23.0 grains of 4198 for a mv of 1500 fps and me of 1308 ft lbs.
A 260 grainer (same bullet) with 35.0 grains of 3031 for a mv of 1480 fps and me of 1263 ft lbs.
No primer was specified for any of these loads. I'd recommend using a standard large rifle primer and backing off the above loads 10%, then working up.
From a power standpoint, the 40-60 WCF has a bit of an edge on factory .44 Mag ammo of comparable bullet weight in a revolver, but is inferior to the same ammunition in a rifle.
If you are shooting this in an original Winchester 1876 or a clone, keep in mind this action is the toggle-link type, it is relatively weak, and it definitely does not work well with hot loads, at least not for very long.
Speaking of 45-70's though...my Marlin 1895SS is one bad cat..my favorite deer load is 51.0gr of Reloader 7 in Federal brass over a Remington 300gr hollow pt...one hole cloverleafs for 3shots and its running a hair over 2000fps..every animal hit with it has dropped on the spot..I really like putting a big hole in something and not tearing up the surrounding meat
My 45-70 reloads have been pretty much restricted to blackpowder. I love using a massively heavy bullet in heavy brush, and I love using blackpowder. The old fashioned way.
The Marlin action is too short to use a 500 grainer, though, because it's built on the 336 action, which is shorter than the original 1895 action. So I've ben using factory Federal 300 JHP loads, which do 1900 fps in my Marlin. The 500 grainers are used in my Browning 1886 lever action rifle and carbine, plus some single shot rifles.
I loaned my Marlin 45-70 to a friend who took it on a bear hunt in Canada. As you said, dropped on the spot. Everything I've shot with it has dropped like it was poleaxed. I can't say the same for some of my modern cartridges, even though on paper they put out a lot more energy and in theory are superior.
I champion black rifles here, and own and use a bunch, but my favorite rifles are lever actions and single shots from the old west, and comprise the bulk of my collection.
Me too Brian...I love my Marlins..the 45-70..444Marlin and 375Win are favorites..the 356Win and 38-55 in a cowboy are in there too ..I have a Mod 71 Winchester 348win but never hunt with it as it was my dads and its pristine from 1955