In my opinion 38/55 is as large as you can go.  CPA will chamber for 45/70 but it's a larger and stronger action than the origionals.

Boats

CPA told me they don't advise any bigger than .38-55 on Stevens 44 1/2 actions. I considered a .40 cal like .40-50 SS or .40-60 Maynard on my 44 1/2 but was advised against it.
I personally feel that as long as "black powder cartridge" is the goal and black powder pressure levels are adhered to a .40 cal on a case the dia. of  .30-40 Krag that it would be plenty safe. There is the concern about what someone else MIGHT do with such a chambering. The only real safeguard against such concerns is to make every ctg so small or every action so large no fool can possible overload. As Bill Ruger once said, " There is no proof against fools."