Wouldn't a slightly shorter case produce the same result?
The 45-70 has a standard size brass case with a standard cartridge OAL (overall length). It was originally a blackpowder load, thus the powder was more bulky and filled the case. Modern smokeless powders e.g. H4198 are more compact.
You'll note that the shorter case approach is just what the WSSM short magnum rifle brass is designed to do. Keep all the powder down by the primer with a shorter, larger diameter brass case.