While I've had experience with a friends Sharps and Winchester .45-70, mine have been more modern guns. I had one of the 1895 Marlins, the regular one, before they got guide gun short or cowboy shooting long. Good gun, had it for a long time, used an old Weaver 2.5x Post scope. Shot 385gr cast RN up to 1800fps. Kicked on both ends. Don't crawl on the stock!!
Also had a Siamese Mauser, with a lovely 26" target barrel. Cool gun, weighed tons.
My favorite rifle is my Ruger No.3 in .45-70. Bone stock other than a good thick recoil pad, and 2.5x post scope. Accurate, and can take the heaviest loads, better than I can.
Have shot the Hornady 350gr RN at 2200fps! MAX LOAD!! REALLY kicks on both ends.
The Hornady slug holds together well even at that speed.
Do be aware that the Speer 400gr JSP is not suited for high speed. It's built to give controlled expansion at regular .45-70 black powder velocities, and a bit above, but if you push it to 1800 you will likely have core and jacket separation. Saw one deer that happened to. The bullet "failed" but did anchor the deer.
I know that if you push that bullet even faster 2100fps out of a .458 Win Mag (and that's not max for the .458) the Speer 400gr JSP is overdriven and acts like a good varmint bullet, expanding rather explosively.
SO,if you are going to load the .45-70 heavy, in a suitable gun, be sure to pick suitable bullets for the velocities you are going to use. Not all of them are.