Hello,

I recommend to buy a surplus military extractor and assemble it to the bolt, for it has already the correct dimensions.

The matching-numbers original extractor should be kept in your spare parts box.

Your proposal of welding and heat treatment(where to locate/buy the welding wire with the correct material?) seems to be too complicate.
The filing is not the problem if you work careful and have a spare extractor for 8x57 IS at you bench, where you can take the correct dimensions and the contour.

regards
Rolf

PS: Maybe lancaster can give you some more support, he is a real bonanza for such information
:-)