Hey Cosmoline,
Do not be afraid of the Marlin 1895 Cowboy being able to handle the hot stuff. The octagon barrel does nothing to weaken this rifle; it simply makes it look beautiful. The Marlin 1895 action is the same whether it is chambered in the 45-70 or the new Marlin 450 Mag which is touted for its hot loadings. I have a Cowboy in 45-70 with a 26 inch octagon barrel, and it is simply a beautiful piece.
Remington and Winchester 45-70 ammunition is made at relatively milder loadings so it can be used in the Springfield Trapdoor whose action is not nearly as strong as the Marlin 1895. The Ruger action is actually stronger than the Marlin 1895, but not by much.
The Marlin can hold ten 45-70 rounds in its full length magazine. The Ruger - well you get the picture. The Ruger will shoot 500 grain bullets, but the Marlin action will not function with cartridges over a certain length which I happen to forget what it is at the moment. For me, this means the heaviest bullet I will use is a 405 grain flat nose in either jacketed or cast. Some folks have found heavier bullets that they are able to keep under the over all length, but I have not chosen to try it. I am happy with the 405 grain bullets.
Since I load my own bullets, I am able to make ammunition that is just as hot as the 450 Mag. Actually, there is no real difference between the 450 Mag and the 45-70 except the 450 Mag is belted and will not chamber in a 45-70. That way, one can not be fired in an old Springfield Trapdoor or one of the other weaker actions. Both rounds, however, are fired in the same basic Marlin 1895 actions. To tell the truth, I do not like shooting those really hot loads. A 405 grain bullet over 50 to 55 grains of IMR 3031 really thumps me a lot harder than I care to take. I'm not hunting Grizzly - mostly punching holes in paper, so I have settled on the milder loadings similar to Remington and Winchester factory loads.
I installed a Lyman #2 tang sight on it, made and tooled a scabbard for it to match my holster and cartridge belt for my Ruger Super Blackhawk, and I think they just look fine. The long octagon barrel handles just fine for me, and it feels good.
Again, Cosmoline, do not worry about the strength of the Marlin 1895 action and the Cowboy model in 45-70. It is an extremely strong action that will handle ammunition hotter than most folks care to take as a steady diet. If you like the Cowboy, by all means, go for it.
Best wishes,
Dave Wile