last fall I bought a Flat spring Hiwall off this forum and a Badger barrel off another(project that didn't get to) and took all to my gunsmith because he had a reamer to chamber a 40/70SS. Bought some Captech brass and we discussed how to go about modifying the chamber to fit the brass. His was stretched 30/40 so rim and base was a little different.
Been shooting it , it's very accurate. Now the 2 part, took some 405 Hornaday brass and trimmed them to 2.5 and ran them down in my full length 40/70 RCBS die then trimmed them again, Presto, fit perfect. The thicker brass came out to .4005 to .401 and perfect paper patch brass.
Dropped 74 grains of Swiss and an LPDE wad , compressed.080 and seated the bullet, took it out on the deck and shot it.Worked great

When I had John Taylor lengthen the stubby 22" barrel on my Hepburn back to 30"; he asked what bras I planned to use. I told him I was going to use .405 Hornady brass, so he cut the chamber for a shortened and sized .405 brass. 
It's the best decision I ever made, and glad he asked! The forming, and firing of the  reworked .405 brass is simple, and shoots great with the TJ's 1-16" liner he installed!

Re. use of the excellent Hornaty 405brass:
Other than "tradition" for the actual 40-70 SS at 2.5",
its more practical to simply chamber for the 40-72W
which is the same length as the Hornady 405 brass.
as in " why should you have to bother with trimming off that
approx. 0.080" or so to get down to the 2.5"
beltfed/arnie

That's the 'game'.

I have a rifle that isn't marked.

So it's either a .375 Win or 38-55 depending on which is PC.

<cringe>

Good choice on the groove diameter - lots of moulds out there.