I found that GOEX "Cartridge" is pretty close to old-timey powder. I prefer to load Pyrodex P or RS, since it is a bit easier to clean up. I am also using a modified Lee 50-70 mould cut down to 350gr, which leaves more case capacity for powder instead of lead.
I can put 45gr of Pyrodex P or GOEX FFFg/Cartridge in there with no problems at all and just a slight amount of compression. (1/8" is what I have measured)
When I switch to magnum large rifle primers, I am getting closer to the specs for what the original shooters used.
(there was so much mercury primer powder in the rims, it acted like a "duplex" load we see today)
[I have not tried a duplex load yet...this summer, I might take 4gr of Unuque behind 40gr of FFg and see how it goes. This is based on the 10:1 duplexing method some BPCR shooters swear by]
I was told by another Spencer "archeologist" that older Spencer ammo from UMC was easily capable of 1100fps with a 350gr bullet. (or there abouts...)
Each manufacturer used different components.
Winchester tended to use good powder and loading methods for ammo. (overall)
Some of the other manufacturers just took what ammounted to "military surplus" and worked with that.