I think you've got that backwards. As the owner of two Shiloh 50-70's (one a carbine), one Shiloh Business Rifle in 45-70, two .50 Shiloh '63's (one a carbine), a 45-70 Rem RB, a .43 Spanish RB, two H&R Trapdoors (one a carbine) and a Browning '86, I beg to differ .....
When I was casting about trying to decide what calibre to order for a carbine, I spoke to the late Dave Higginbottom(sp.?), founder of Lone Star Rifle. His advice was a 50-70.
Why? the same amount of powder and lead that would rattle your teeth in a light 45-70 is tame in a 50-70. Why? He explained it as simple ballistic physics - the greater diameter = less pressure = less recoil.
Your comment cannot be based on experience.
I would recommend sticking with a cartridge based on the .45-70 head, rather than the .50-70 head, as the smaller diameter means less backthrust on the breech, though the difference isn't significant. (Recoil will be less with the smaller round.) Best of luck with your project.