How much of the lower lug abutment was removed? I would much rather err or the side of safety..
i had to remove right at .187" so i'll just call it .200", the back wall of the receiver also has to be flattened out before you can bolt on the bottom metal to scribe around the inside of it so you can see exactly how much metal has to be removed and where the rails need to bump out for feeding. i work the right side rail first then the left. before working on the rails i would suggest holding a standard action upside down and pushing a few cartridges down the rails so you can see exactly how they move the cartridges to the center and up the feedramp. i use a carbide burr to shape the feedramp followed be sandpaper wrapped around a dowl. i set mine up to feed round nose bullets but if you set it up for 270gr. spire points you will be remove a little bit less material from the feedramp.
its best to cut the rails with a ball end mill on a mill but i have done it with a carbide burr on a drill press with a x-y table. either way if you don't understand what it takes to make it feed its best to leave it to someone who does.
as far as building a 375 on a magnum length mauser action i consider it a waste of the action, i would do a rigby, gibbs or nyati on one of those or something that takes better advantage of the action length.
i have a very nice 35 whelen on a 1909 oberndorf action, it & my 375h&h are my main two rifles.
i also have a 375/06 reamer sitting in my toolbox waiting for one of my 1903 actions, it will be built in the wundhammer style