CAD work is now complete. The new bolt head has been modeled with the correct bolt face diameter for the cartridge. It will accept off-the-shelf Sako extractors, and work with an unmodified "Remage" bolt face, as shown. All that will be needed to convert a Mosin to another caliber with this bolt is to take any Remage barrel, turn the appropriate .975-16 threads on the tenon to properly thread into the Mosin receiver, and headspace it using the "Mosage" barrel nut which I will also be machining.
Next step: 3D printing the parts to test for fit and function. I have an SLA printer which should give me sufficiently accurate geometry to determine whether the design is ready to be machined. I will also order a custom step reamer for the firing pin channel, because off-the-shelf tooling could not be readily adapted to work efficiently.