Hello Bonanza,
I've got a couple of Ernie Stallmans barrels & they are wonderful - however I'm not sure I can say the same for his advice to you regarding smokeless powders in BP proofed guns.

The fact that correct 'Nitro for Black' loads are not only safe (assuming a gun that's in sound condition) but are also 100+ years old in concept seems to have escaped the notice of some 'experts'. Perhaps this is because many 'Nitro for Black' loads were loaded by British companies for British guns & used mainly in Commonwealth' countries.
However as pointed out by others the current factory 45-70 loads are 'N for B' loads.

BP is as you said very forgiving. Loading smokeless powder even for a modern gun requires far more care & has far more variables. Nitro for black loads are simple to make up but do need care in the research of the correct load. This does not mean though that smokeless cannot be used in a BP gun.

Perhaps the most foolproof powders ever produced were the old 'bulk' powders - all the advantages of smokeless but also the ease of loading of BP (I wish they were still around for certain purposes).

It's been shown often enough now that the pressure curves of BP & that of a suitable smokeless powder for the job in hand are quite similar. All a barrel knows is that there is a certain pressure at a certain point. The barrel does not care about the chemical makup of the powder that produced that pressure.

Most original BP loads for double rifles produced around 10 tons peak pressure. The original N for B loads produced the same pressure or normally less. The modern N for B loads (like those quoted in Graeme Wrights book) produce under 10 tons - more like 8.5 tons on average.

Regards
Webley