Don: I have hand loaded black powder in my .45-70 several times. With that cartridge I load 65 grains of FFg (light crush load). (It's not 70 grains because modern .45-70 cartridges have a lot more metal around the base than the original thin walled cases had.) There is a lot of data about loading that cartridge and the .43 Spanish (which I also have loaded) available.
With loading any of these old guns there is a chanch of an accident. Having said that, IMO you can't load enough black powder in a Rifle Shell Casing to cause unsafe pressures in a good gun. I personally don't like shooting BP in cartridges because you have to wash the cases to prevent corrosion from weakening the wall. Cleaning 1 or 2 rifles is OK but to have to clean 40-60 cases is a PITA.
Shot shells are something else, and as fussy as they are with their wad column height etc I don't bother with them. If you do decide to load shot shells though the Dixie Catalog table for BLACK POWDER LOADING DATA FOR BREECHLOADING SHOTGUN SHELLS says a 20 gauge uses 75 grains FFFg with 1 oz shot in the 2 3/4 inch shell. They don't give data for a 16. They also go to FFg for the 12 gauges.
If you know the "DRAM" loading of the shells you you want to load you can calculate the grains of powder by multiplying by 27.34. (3 DRAMS=82.0 grains).
By the way, if you see a Dixie catalog for sale, even if it's out of date, Buy it. There is a LOT of good information in the back. It's worth ordering it from Dixie ($5.00 + postage) if you can't find it at a gun show.
One last thing about old strange calibered rifles of long ago. The price of new cartridges can be truly amazing (if you can find a supply). A .45-70 cartridge costs about 45 cents each BUT the cartridges (unprimed) for my .43 Spanish cost about $2.00 EACH! (They do amaze the people at the range though because they look like the Biggest .30-30 case you've ever seen!)
Have a Good un