Hello, Sixtus:
The Armisport/Chiappa Owner Manual, which can be downloaded, contains info that requires only commercially loaded ammunition that conforms to European Union or SAAMI specifications for the 56-50 cartridge. SAAMI's latest 56-50 specifications are no longer available. The EU ones are open to interpretation. There have been commercial 56-50 ammo providers, but you will have to locate one. That can be tough.
As Taylors told me: "If one uses handloads, they are on their own."
Black powder loads of slightly compressed 3Fg and a 350gr. bullet achieve roughly 900 -1,000fps.
I doubt using a heavier or longer than 350gr bullet would allow 35gr 3Fg, so velocity should be less, presuming the round would feed.
There are some threads here that include smokeless powder loads and velocities. Launching a 450gr bullet at 1200fps puts it into the 45-70 recoil range. Stout for sure.
One alternative to tinkering might be to offer to buy a fully functional one from a member, here.
The 24" twist should work with the heavier bullet, if velocity can be achieved.
Just my respectful, two-cent opinion:
Trying to make a Civil War, black powder design into something it was not, nor could be upgraded into after the War may be pushing the limits.
All the best,
El Supremo/Kevin Tinny