I'll be able to provide 375 Ruger case lengths tomorrow. Both of unprimed Hornady casings sold for reloader's use, and of loaded factory cartridges.
Below are two drawings that I generated, following completion of some mathematical calculations. The first drawing shows the parent 375 Ruger cartridge overlain on the proposed 30/375 chamber.
Both drawings were drafted in the effort to predict the case neck length that would result if the shoulder on the 375 Ruger case remained at 2.180 inches from the case head, and if only the case shoulder and the case neck were reformed, preparatory to fire-forming in the chamber of the rifle.