I've formed brass for 11mm Werndl (which has the same general shape as 45-75) from .348 Winchester. I found that annealing the cases beforehand, then fireforming with light loads of pistol powder under a case of inert filler (I think I used worn-out tumbling corncob--I know, it probably wears the bore, but on the other hand, perhaps it helps lap out some of the pits, too!) worked well. I compressed the filler over the powder and sealed with a lump of candle wax. With really light loads, the case neck would be undersized and the case body would be wasp-waisted. As the fireform charge was adjusted upward, the neck would be increasingly larger. The last part of the case to form as the charge was increased was the middle portion. With cases that "almost" form with a fireforming load, I sometimes go ahead and reload with a light bulleted load to fully form the partially-formed case.
Without annealing the cases it just didn't work as well--it was harder to form the cases, and I had numerous losses due to splits in the neck and shoulder. I haven't formed .45-75 cases yet, but plan to start the same way, although I'm tempted to work out a way to fireform using the sizing die so as to reduce wear and tear on the gun. Also I haven't checked to see if I need to make adjustments to rim diameter and thickness.