Good points on the bores of the conversion guns. You can also generally see the braze line at the muzzle when they relined them.
When loading using the simple method mentioned above in an original 1886 Winchester with fresh rebore from 40-65 to 45-70, I was getting about 4" groups @ 100 yards with casual rest, probably over the seat of my motorcycle or hood of the truck, we had no formal range, just cinder pits to shoot at. My lube was the older Lyman "Ideal" lube, a black substance that came about in the early 1900s and was supposed to be suitable for both smokeless and black loads. I dont recall much info being commonly available in the early 1980s about black powder cartridge loading, what i was doing seemed to work fairly well all things considered, and when I load more black in cartridges, it will be basically the same methods, but with SPG lube.