Sanding copper washers seems like a good plan. Good corrosion resistance and good sealing ability.

If you cant get different shims; You could chuck your pre chambers up in a lathe and face a small amount of metal off the sealing surface making them screw in more.
With little math with the screw pitch you can calculate where the glow plug holes end up.
Ultimately the holes should be indexed judging by tour picture in the 5:30 to 6:30 area with a torque of 200 pound feet. This area will clear the injector lines.

if you use copper washers dont forget to anneal them so they bed in properly as copper work hardens.