Above are two candidate magazine wells. Top one is Polish in origin bottom is a Russian Izhevsk. Surprisingly the Polish magazine is narrower, and will not accommodate the rim of a .45-70 cartridge. The Russian magazine is wide enough in the rear, and middle sections to fit the .45-70 round.
From the above picture it clear that for the Russian mosin magazine only the front third needs to be opened up, to loading. Voland believes that the latter is best achieved by cutting out the front 3rd of the magazine sidewall, and rewelding it to fit the .45-70 cartridge (See figure below).