Smokinjoe has the source I'd use. The .45/100 Winchester was the 2.4" case, loaded with a 270gr. bullet. With 300gr. and slightly heavier bullets, the load was reduced to 90gr., hense the .45/90(2.4") The 2-7/8" case is a Shaprs case and was loaded with from 100gr. to 120gr. of powder, with bullets up to 540gr. If you call a Sharps round or rifle a .45/120, you are actually talking about the 2-7/8" case - historically speaking. Only modern gun makers and writers have changed the meaning (as they do sometimes through ignorance) to be a 3-1/4" case. Only Winchester, the English & custom gun makers in the US chambered a 3-1/4" case in .45 or .50 Sharps rifles - never Sharps, as I understand the situation.
: A standard .45 x 2.4" is probably about all the performance one can utilize with smokeless powder at low pressure, without having to use fillers & this only with the slower burning (intermediate) powders. This case will give over 2,000fps with a 400gr. bullet at 30,000psi - Need more? For what? I think you'll find the recoil about all that's comfortable. A longer case will not add an appreciable (noticable) amount of performance but will add recoil, which is the judge of power by some. The longer case will take more powder just to match the 2.4", yet not exceed it by more than about 100fps. 2,050fps is the velocity for a factory loaded 400gr. .450/400 British round; These ballistics worked on all species of game in Africa, quite well. If there is a need to exceed that, I am unaware of it.