I'll just post,rather than E-mail.
To get loaded ammo,or ready to go brass,I'd check with Buffalo Arms.They have a site,or you can call them.They offer a number of versions of dies for reloading.
Some good news is Hornady makes .405 Winchester brass.It is a very good case to make 40-70 SS from.
I do not know your skills and experience,but,you will need to slug your bore.The old rifles were a bit variable.It may have had a .406 bore new,but who knows what it is.This is implortant for what bullet you use.I am not the guru,but groove diameter to + .001 is close for your cast bullet.do not consider jacketed,old barrel steel is soft.
You wil want a cerro-safe chamber cast made.The .405 brass will have to beshortened a little,about 1/16 in(i am doing this from memory,i do not have drawings in front of me).They need to e shorter than the chamber,but not much..005 is not bad.
Odds are good the rim thickness may need to be adjusted,too.Seems like a little will need to be faced off.
I'd suggest getting Venturino's book on "Shooting Buffalo Rifles" and the SPG Bullet lube booklet.
I suggest black powder,read those books,and you will learn what you need to know.If a person reads Paul Matthews books on black powder cartridge reloading,there is a wealth of experience in them.
That sounds like a fine rifle!