If it's safe with 70gr. of FFg, it will be safe with the same volume of pyrodex. Pressure-to-pressure it's a safe load.
Couple of things to consider. Old rifles often have over sized bores so it's worth slugging/measuring the bore...one 1883 dated Rolling Block has a .461" and my firend's trapdoor runs .460", but have other people's numbers that show anything from .457" to .463" isa possible. BP usually will usually "bump up" a soft alloy bullet well and shoot accurately; hard alloy bullets often just key-hole.
Other thing to know is that while Pyrodex cases will need a trhough soap-and-water cleaning and brushing...and even then, better to check them in a day or two to see if they are growing green fungus.
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Not enough data, but fo far tht only substitute powder that is giving consistantly much higher velocity than expected is Triple Seven. I don't believe there is a free lunch...higher velocity is going to have to equate to higher pressure. May not be high enough to damage an old gun (and trapdoors are evidently a lot stronger than their reputation), but I'll be conservative.