Welcome. I built my 404 on Czech 8mm 98 Mauser. Your 7mm RUM will require little or no work opening the bolt face. I suspect the action would be long enough but maybe check on that. Push feed would work fine for DGR. Just make sure to change the extractor. I don't care for Model 1917 cock on closing but I guess that can be fixed. Guppy belly magazine also doesn't turn my crank. My go to deer/elk/moose/plains game rifle for sixty years has been 03A3 Springfield. It is the smoothest cycling of the three military donor rifle designs (I also have a P14 gathering dust in the vault). I think the bolt is wide enough and action long enough but you may want to check on that. Unfortunately, finding aftermarket big bore bottom metal for 1903 could be problematic. I used 416 Rem bottom metal for my 404 build on 98 Mauser and it worked without modification. Good luck.
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404 in Africa last month.
The 425 WR could be problematic. The original design was indeed straight wall magazine to employ stripper clip loading. Eventually WR attempted staggered magazine with limited success originally. Standard action Mauser follower springs could be "saggy" especially over time. 425 rebated rim requires a robust follower spring. I would advise exploring the robustness of the follower springs of your three candidate donor rifles before leaning in that direction. Also, while it is possible to make 425WR cases from 404, finding the latter is not easy and in at least some African countries ammo case heads are inspected at customs to ensure they match the rifle.
The .425 Westley Richards is sometimes referred to as the Westley Richards .425 Magnum Express. It became a much respected cartridge, favored by the legendary Frederick Courtenay Selous and by John "Pondoro" Taylor, author of "African Rifles and Cartridges".
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